So You Want To/Write a Real Time Strategy

WAR! Politics, bloodshed, heroism, betrayal, religion, luck, and pure cussed skill: there are very few topics that capture the human heart like war. So, of course, there are games about it.

Make sure to look at So You Want To Write A Story for good general advice, and it might be a good idea to check out the other gaming genres on So You Want To for other ideas as well. (Much of this is being reproduced and inspired by the Write An RPG article.)

Necessary Tropes
Remember the Rule of Fun. There's a huge grab-bag of tropes concerning Real Time Strategy: Construct Additional Pylons, Arbitrary Headcount Limit, Ridiculously-Fast Construction, and others that are covered on the main RTS article. All of these are considered Acceptable Breaks From Reality, because they make the game fun, and you have to be willing to pay the piper in that regard. You could craft the most intricate and convoluted story or battle system imaginable, but the gaming industry is littered with great ideas that never caught on because the game they were packaged in just wasn't any good. Sometimes those creators get a second chance. Sometimes they don't. (Remember also that a game's complexity can work against it; see basically any Turn-Based Strategy game ever for a reminder to Keep It Simple Stupid.)

How innovative are you planning to be? Some people want to break the rules and do things new. Others are just looking for something to tide them over to Starcraft 2. Remember also that innovative games are a bigger gamble: either you're going to kick Blizzard's ass or you're going to be an epic failure, but there's no safe, reliable in-between. That's reserved for Roguelikes, Pokémon clones and other Follow the Leader games.

You're at war, so some War Tropes will be appropriate. Likewise, some Death Tropes will probably find their way in.

Choices, Choices - Gameplay
Here's where things start getting really complicated.

What sort of setting are you looking at? As long as there's people, there's war, so you can have stick-wielding cavemen, high-tech futures with robots and lasers, magic-based fantasy, or anything in between. (Heck, Rise of Legends did all three, though with Da Vincian steampunk instead of cave thugs.) You could even go supernatural: zombie invasion? Vampire invasion? Werewolves? Mutants? Ninja Pirate Zombie Robots? (Calvin and Hobbes once had Tyrannosaurs flying F-14s. Calvin: "This is so cool!" Hobbes: "This is so stupid.") Also, what's your battle plane? Are we talking ground-based armies here? How about interstellar fleets with full 3D maneuvering?—it's a known fact that the Homeworld fandom is looking for love. What about something in between? How about both?

Would you like your gameplay to emphasize macro- or micromanagement? The former is best exemplified by Total Annihilation and Supreme Commander, both of which are the brainchildren of the same guy (Chris Taylor), and both of which allow you to command thousands of units at once. The latter's adherents currently play Warcraft III, in which an army of 50 units is considered quite large, and you have to be able to watch over all of them, activate their abilities and keep them healthy. (If this sounds difficult, it is, but War3's popularity suggests that people like it anyhow.)

What resources are in your game? You Require More Vespene Gas gives the overview, but most titles in this genre go with basics: Gold, Lumber and Population. Others have tweaked the scale: resources you might not need, tons of resources, only one or two... This has an impact on gameplay, remember. For this troper's money, Rise of Nations proved that resource diversification makes things way easier, but maybe you don't want it easier. Maybe you want The Player to think long and hard about every build order issued. It's Up to You. For that matter, what if you want The Player to be able to de-emphasize resourcing? How about building a race into your game that doesn't resource at all, thus depriving opponents of a vital weak spot to hit?—though at the cost of some other weakness, of course. What if each faction in your game needed extra amounts of a certain Lumber-style resource? This would force each faction to stick to certain locations on the map, where said resource is prevalent, and thus have an effect on their playing style.

Any extras you want to throw in? Squads of infantry instead of individual soldiers? Support Powers? A tech tree? RPG Elements and persistent troops that gain experience points? Hero units? Terrain bonuses? What if you were to splash in 4X traits, like Sins of a Solar Empire has? What about some Collectible Card Game flavor, like Pox Nora? Courts-martial if you screw up really badly? (The Wing Commander flightsims had these as a Nonstandard Game Over for screwing up plot-critical missions.) Something else entirely? The sky's the limit.

Competitive vs Casual Play
There's one more thing you need to think about, and this is a big one. Do you intend the multiplayer component (if any) of your RTS to be for competitive play or for casual play?

The answer to this question will fundamentally impact your design. It's not necessarily that the two needs conflict, though they do create friction against one another.

A competitive game needs to have a deep skill set. It needs to allow better players to win over less skilled ones at least 70% of the time. It needs to have strong time pressure, requiring the player to do more things than the player is capable of. Competitive games tend to make their players uncomfortable, pushing them to do more than they are currently able to do.

Casual games need to encourage the player to continue playing. It needs to reward the player's investment in playing the game. These can be tangible in-game rewards like money to buy units or whatever. Casual games need to encourage play by removing elements that are repetitive or thoughtless.

These two kinds of games are not mutually exclusive. However, repetitive/thoughtless play is a big part of why Starcraft attained such depth of skill. By essentially removing actions from the game (you have to do X every Y seconds or you lose), it forced players to have to do more in less time. That's not the only way to achieve that depth, but it is a way and a way that works. It just so happens to be a way that is counter-productive to casual play.

Races
Are you going for Cosmetically Different Sides, or would you like separate factions with Competitive Balance? The upside to having separate factions is that gameplay is way more interesting, since each side has different units, strategies and tactics—and "more interesting," if done properly, means more fun. The downside is that whole "Competitive Balance" thing; Faction Calculus is a lot of work. The poster-children for this trope are Starcraft and a non-RTS title, Magic: The Gathering. Blizzard has patched Starcraft twenty-two times thus far and it is still not balanced (though it's close), and Magic is far from balanced either, though to be fair Wizards of the Coast have released over thirty Expansion Packs for it. The point is that separate, non-identical factions increases the likelihood of a Game Breaker unit or ability being discovered... And, if your RTS gets popular (like you want it to), your players are going to be scrutinizing the game with microscopes to find that Game Breaker so they can use it. Expect them to find the ones you tried to disguise. Hell, expect them to find ones you didn't know were there. Long story short: Competitive Balance means more fun, but also more work. Possibly a lot more work.

Alternatively, if all sides are essentially identical, you will likely have a more serious competitive problem. One of the reasons Starcraft works competitively is that each player, no matter what race, must have a plan to deal with each other race. And that plan will be different for each other race. So to be skilled, you must practice at least 3 other matchups. You must know what not only your units and mechanics do, you must know what your opponent's units and mechanics do. Not only must you know the Metagame for your race, but you must know what the prevailing strategies are for your opponent. This increases the skill set required, thus again adding to skill depth.

In competitive Starcraft, mirror-matchups (same race vs. same race) tend to be the least interesting matchups. Both players do the exact same build, use the exact same units, and the game is generally over in 10 minutes. Whereas non-mirrors tend to allow greater diversity of tactics and styles. People tend to play mirror matches the same way, while other matchups allow players to exercise their creativity.

You should not go the Cosmetically Different Sides route if you're interested in competitive play. If you do however, you need to look at RTS mirror matches and see what kinds of design creates the most interesting matches. In competitive Starcraft, the most interesting fights generally come from Terran vs. Terran (though these can devolve into Tank wars, where nobody moves and it takes an hour and a half of mind-numbing resourcing and building before something interesting happens). Study the Terran race and see what factors allow them to create interesting play. Apply these factors to your game.

Choices, Choices - Story
First, do you want a story at all? The single-player campaign is a staple of most RTS games. But so was the single-player of an FPS for a time. Then Quake 3 Arena came out, without any real single-player at all. You could play multiplayer against bots, but that's about it. And it sold just fine.

People will buy a multiplayer-only RTS if the multiplayer is good. So don't feel that you must make a single-player campaign. Only do so if you really want to tell a story. Otherwise, just make some tutorials to teach people how to play the game, use the units, etc.

Most RTS titles involve The Player acting as a Non-Entity General, which is a sound choice for the genre: it keeps The Player involved, clear-headed, and in a good position to observe the Wangst and suffering being undertaken by any actual characters. However, there is absolutely nothing to say that The Player must be a AFGNCAAP; Sacrifice takes a 3rd-person over-the-shoulder view of your wizard. And there's something to be said for getting The Player involved with his character, and maybe even his men if possible. Of course, the line must be drawn somewhere, especially if you plan for the average player's force to consist of hundreds of units, each of which your game would have to name, randomize and personalize. But the point is, you don't have to skimp on the characters. And you shouldn't.

This editor is a little biased on the topic, but let me say it anyway: the vast majority of storytelling is about characters, and their relationship to each other. And nothing I have seen, in all the fiction I've read and all the games I've played, has ever convinced me otherwise. There are very few exceptions, mostly in the form of travelogue stories—tales like The Wonderful Wizard of Oz or Gulliver's Travels where the setting itself is practically a character in its own right. ...And yet even that seems to affirm my statement: the land of Oz, and Dorothy's relationship to it, is the most important character in the story. And, in terms of storytelling, Real Time Strategy is no different. The good ones may have great gameplay, but the best ones also have great characters, characters we care about and whose fates we want to learn about. They may not be the main reason we play... but they're definitely up in the top three. And the games that don't have characters... Well, there's only two reasons to play them (Rule of Fun, and online competition with your friends). That'll get you some mileage, but not as much as it could have.

Your setting will have an effect on your plot and characters too. But it might not be as big as you think. All the best stories deal with universal and human themes—life, death; building, war; love, hate; hope, despair; betrayal, redemption, etc. You can set those themes against any milieu and they will still ring true. Try to fit it all together elegantly, and if your game's setting can influence its plot and characters, that's all well and good. But setting will probably have a greater effect on your gameplay than on your story. Star Wars isn't famous because it had lightsabers, it's famous because it transcended lightsabers.

Female protagonists are underused. Having said that, you're writing a war story. In Real Life, there are good, meaningful reasons for why most human cultures send men off to war instead of women: once a woman is pregnant, it isn't strictly necessary for the man to stick around anymore, or even be alive. Plus, you want a big next generation, to compensate for the fact that some of the current generation is currently out getting swords shoved through their faces. Finally, imagine two tribes that send half their population off to fight a war; Tribe A sends all the men, Tribe B all the women. Both armies are wiped out with only a single survivor remaining: Tribe A now has one guy with 50 wives, while Tribe B has one woman with 50 husbands. In ten years, which tribe is likely to have more children? These are the tried-and-true historical reasons for why women don't fight wars, and you aren't going to be able to contravene them for your story.

But wars go badly. Cities are sacked. Women are raped. The line between combatant and civilian gets mighty blurry at times. If you wanted to get your woman-heroine into the war, this sort of last-ditch city defense would be a good place to do it: she has a legitimate reason for being on the front lines (she is the front lines!), and can thereafter join the military proper, having proved her competence as a commander. You also get to play your Double Standard cards, depending on your story's culture and time period. Alternately, just have her Jeanne D'Archetype it. Or, even more alternately, just ignore the double-standard thing entirely. A lot of fantasy games play the gender-equality-in-war card without any shame, so frequently that it probably belongs in Acceptable Breaks From Reality; and, in a present-day or future setting, you have the dual excuses of women's lib and population surplus ("We have enough babies, so we don't need yours"). There's also the possibility of removing pregnancy entirely from the equation via Applied Phlebotinum. You can get around it.

It's Up to You is a huge part of, well, most Video Games in general. However, it is not a huge part of most wars. Most soldiers only see their little part of it and don't have many chances to singlehandedly change the course of a war. In most campaigns, there are instead multiple commanders fighting on multiple fronts with multiple goals to achieve. Ever thought about playing up this part of it? The otherwise-unnotable flight sim StarLancer did this by providing between-battle news reports, giving you the sense that you were just one cog in a much larger war machine. Frequently you'd hear news reports about a squadron long before you actually flew with them, which increased the sense of "celebrity exposure": "Whoa, I'm flying with The Ace I just heard about on the radio!" For that matter, why not give players the chance to affect multiple fronts? The (again) otherwise-unnotable Third-Person Shooter Win Back 2 put you in charge of two-person infiltration missions. You'd play as Agent #1 and get chances to accomplish things outside your actual mission objectives. Then you'd play as Agent #2 and receive bonuses at Event Flags when "Agent #1" "assists" you. Let your player play as multiple commanders and then wind it all together as the game comes to a close.

You might want to consider the tone of your story. Blizzard titles especially seem to take place in a World Half Empty where Rocks Fall Everybody Dies on a frequent basis. This got especially bad in Warcraft III, where characters would die only 15 minutes after their introduction and long before any pertinent characterization could be attempted. Instead of making the war seem tragic, this Kill'Em All attitude just makes it seem like your enemy is faceless and meaningless; the attempt at humanizing the enemy completely backfires. That's not to say that characters shouldn't die in a war; leave too many heroes alive and you strain Willing Suspension of Disbelief (the series finale of the new Battlestar Galactica suffers from this). In the end, war is a pretty bleak prospect... But just how much do you want to play that up?

Pitfalls
Keep an eye on your GUI. A game is only as easy to play as it is to control. In RTS that's less of an issue (especially if it's a computer game), but it might be a good idea to review the Stock Control Settings for the genre, as well as catalogue any improvements on these schemes. The Sims, which is basically a RTS without war, gives you a visual queue of a character's not-yet-executed orders, which you can then modify on the fly. Supreme Commander lets you issue unit orders to a factory, so that any units produced there will come out with that queue already assigned. War3 gave you auto-casting on some spells; unfortunately, toggling auto-cast spells didn't come along until League of Legends, which also gave you orange highlights to show when an enemy was targeting your One-Man Army, and—best of all—an auto-reconnect function, lag meter and framerate counter all built into the GUI. Rise of Nations is the king of of innovations: an entity's control palette is always bound, positionally, to the QWERT, ASDFG and ZXCVB buttons, meaning you can keep your hand there and always have full control over any unit or building you select. Pressing Tab lets you cycle through all available research. Pressing Home instantly finds all units, anywhere on the map, of the type(s) you currently have selected. All these make the game more fun—or, at least, make it easier to concentrate on playing, since you don't have to fight a twiddly interface. So check out these UI innovations. You might want them.

If you have spare money, work on your Pathfinding AI. These often suck in RTS games—not necessarily because they're worse than in any other game, but because pathfinding standards in RTS are higher. If you have time to actually develop pathing routes the way some First Person Shooters do—where you actually embed invisible guide rails into the map—maybe you should do so.

As mentioned, RTS titles come with a tremendous amount of baggage in terms of Acceptable Breaks From Reality. If you include any of them, some of your players will detest you. If you subvert any of them, some of your players will detest you. You can't win, so you need to decide—preferably ahead of time—just what sort of gameplay tone you want to create. Some players are administrators, others generals; some want to get mileage out of a small number of troops, while others want to bulldoze with a Mongol horde; some play to win, others to have fun. Know what demographic you're targeting, and tailor accordingly. (Trying to please everyone will probably not work, though you're welcome to try.)

This is more a personal gripe from this editor, but it's worth mentioning: try to keep your single-player campaign consonant with the multi-player experience. By which we mean: don't introduce (many) heroes, abilities or units that aren't available in multi. We Require More Demographics Research, but my hunch is that many players expect the 1P campaign to teach them how units work in multi, and thus what to expect when they go online for the first time. So throwing in a lot of things that aren't in multi will just annoy them. (Besides, the elements that are removed from multi are inevitably the Game Breaker ones. Why would you want an extra serving of those?)

On a storytelling level, do not succumb to laziness. You have a great deal of work on your plate already: units to design, maps to design, statistics to work out, playtesting to do, blablablah. Nobody will care if you just toss on an Excuse Plot; they're here for the gameplay, right? No, not right. Your characters need to be interesting too. An intellectual challenge is an intellectual challenge: "Can I conquer this enemy base?" Fun, yes, in a Minesweeper kind of way, but without engaging the emotions. When the game involves characters, with interesting motivations, who feel a certain way about this mission and aren't hesitating to let you know it, who are engendering different types of sympathy for different reasons... Well, it's a completely different game, because emotions are involved. And since emotions are what you're playing to, as a storyteller and as a game designer, why would you skimp on this? Spend a lot of time on your characters. If you're not good at that, hire someone to do it for you. But do it. And if you find yourself asking, "But, what kind of story can I tell against the backdrop of a war," well, the answer is, everything. Seriously, what can't you tell against the backdrop of a war?

Finally, be prepared to work your ass off. The market is saturated with half-assed RTS clones right now, because everyone looks at the genre and thinks, "Oh, easy peasy, I'll just clone StarCraft and make a bunch of money." Sure, easy. That's why those clones are half-assed. This editor wouldn't know for certain, but I am starting to think that, where video games are concerned, Doing It for the Art is the only way to financial success. You don't do it to turn a quick buck; you do it to make a good game.

Potential Subversions
Okay, again with the Acceptable Breaks From Reality. All of these are available for subversion. But, are you sure you want to? Tropes Are Not Bad, and (as mentioned) these tropes exist to make the game less tedious and more fun... and (as mentioned), Fun Is Good. But still, possibilities exist.

Ridiculously-Fast Construction, for instance, and its cousin Ridiculously Fast Research That Propagates To Your Units With Similarly-Ridiculous Fastitude, are both ripe for aversion. How about a game in which you get to build structures, but they don't actually get finished until the beginning of the next battle? Likewise, you can send a team of scientists home to research your next topic, and they'll e-mail you the results at the beginning of the next battle. (Of course, if your general moves to a new base next mission, then, realistically, you should lose access to whatever new buildings you built this round. But you might be able to get around that.)

Easy Logistics are a tricky one. If you've played Homeworld 1 or other titles in which you do have to refuel and rearm your units, you know it's freaking annoying and rightly ought to be removed. But what about maintenance? What about non-combat casualties?—wars have been won by both superior tactics and diarrhea. What about unit fatigue and/or morale, like in Dawn of War? What about communication?—even during these days of radio and cellphone communication, it's not easy to guarantee that your soldiers have their orders or that they're carrying them out properly (or at all), and an ambush kind of doesn't work if one flank is late to the party. For that matter, what about intercepting enemy communications? Battles, sometimes even wars, have been won by waylaid intelligence. This is generally incorporated into between-mission cutscenes in an RTS, if it's included at all; but what about making it a part of gameplay?

Real Time Strategy contains two sub-genres. One is better described as "Real Time Tactics," in which the emphasis is on controlling small numbers of units and there is no base-building or administrative detailing. Myth the Fallen Lords, Dawn of War 2 and Defense of the Ancients (and its spinoffs) lean in this direction, with the latter taking it to its most logical extreme: you control just one hero, and nothing else. On the opposite end of the scale is (for lack of a better term) "Real Time Base Building," with lots of structure-placing and administrative choices, but little focus on actual military maneuvers. The most obvious examples here are Tower Defense games like Plants vs. Zombies; the Starcraft II mod "Nexus Wars" in which your entire job is deciding which soldiers to throw into battle; and the "dictator simulator" Gratuitous Space Battles, in which you design ships and determine fleet composition, but don't get to do anything except watch the fireworks once the battle is joined.

Death Is Cheap in a lot of RTS games, at least the ones with Hero units. In Starcraft you'd Game Over if a Hero unit died; but they were really strong and probably wouldn't be in much danger unless You Suck. In War3 you could just rez them, no problem. Instead, how about having Hero units that can die pretty much normally? Once you lose them, they just aren't there anymore—no impact on the plot, but you don't get their bonuses and talents to use in battle. Fire Emblem does this, more or less.

As to the plot itself... Well, the sky's the limit, really. But most RTS titles tend to have a designated Big Bad, someone (be it a race, a culture or a person) who is just bent on bloody conquest and needs to be defeated. Why not get rid of that? Why not a war that's started by a misunderstanding, where alliances and peace treaties are possible—nay, desirable!—but keep getting sabotaged by ineptness, backstabbing henchmooks and other Idiot Ball moments? (Three Is Company, AT WAR!!) Or, how about a war where you start off as the Big Bad, launching an unprovoked blitzkrieg on people who haven't done anything to you? You could put The Player in a really uncomfortable position by getting him to sympathize and identify with people who are unquestionably bad. (Check out Ron Jones' movement The Third Wave or Stanley Milgram's obedience experiment for ideas on how you can encourage The Player to buy into authority.) You're writing a war story, so you're probably going to be on the cold end of the Sliding Scale of Idealism vs. Cynicism, but you don't have to be. Lighter and Softer does exist, and rarely gets any attention from this genre. ...Primarily because wars are so steeped in bloodshed, of course. War Is Hell. But maybe you can make it happen. It's Up to You.

Suggested Themes and Aesops
Well, it's taking place During the War, and War Is Hell. It's really up to you where you go from there—as with most tropes, a lot of War Tropes depend more on how they're played than what they really are. Tropes Are Not Bad. (Or, to misquote what the whore said to the bashful sailor, "It ain't [what] you got, son; it's all in how you use it.")

Potential Motifs
Curiously, RTS titles rarely play up much symbolism... partially because nationalism doesn't emphasize imagery the way other forms of patriotism did. Your average American responds to the Stars and Stripes, not a bald eagle. But you don't have to follow that rule.

Enemy minions are people too. You should be able to hear the lamentations of their women... And then have some guy wind up in the loony bin muttering "he was only 16... he had a mama... and a little dog..." over and over.

Suggested Plots
Look, do we really need to go over this?

Obviously, there's a setting to consider: you'll need to set a technology level; but you'll also need to set a magic level. Remember, the two are not mutually exclusive: some games abound with Applied Phlebotinum, and a few of them actually make sense, whereas others hew to the Low Fantasy model or the deep end of Mohs Scale of Sci Fi Hardness. Higher values on that scale mean more realistic gamelay... And, potentially, less fun. So, think hard.

Set Designer / Location Scout
Just FYI, most battles take place out-of-doors, primarily because there are very few buildings that can hold two armies, and fewer still that would still be standing once the dust had settled. So, by and large, you'll be outdoors a lot. The exceptions are small-unit tactics maps (IE Starcraft).

Use your imagination and make some cool-looking terrain. But remember the operative word there—terrain. Positioning, and taking advantage of the natural features of the earth, is a big deal in warfare. If you create a map with lots of choke points, high grounds, trees and forests for cover, and a mountain that should take a while to climb, but then forget to make those things meaningful in terms of combat mechanics, Willing Suspension of Disbelief goes out the window. Or maybe into a wall.

One thing that is best mentioned here is The Problem with Licensed Games. There are a ton of existing franchises out there which have never received good video-game adaptations. The reason for this is that rules change as you switch between media: what works on page, or on the silver screen, or on the tube, does not necessarily work with gamepad in hand. There will probably never be a licensed RTS for The Wheel of Time, for instance, because its particular tone—impending apocalypse, World Half Empty, damned-if-you-do-damned-if-you-don't destructive savior, politics and subterfuge, the gender gap—is hard to adapt into video games.

Furthremore, if you try to adapt another franchise, you're going to run into its rules, at least some of which will be contrary to good gameplay. If anyone ever tries to make a MMO based on Avatar: The Last Airbender, for instance, they will run into the problem that nobody can play as an Airbender—there's only one left (it's in the title), and because he's the main character he'll need to be an NPC. Boom: a situation where Canon is at odds with good gameplay... and we haven't even watched the show! All we've done is read its title! If you try to license an Intellectual Property, this kind of thing will happen a lot. Either you can do a Pragmatic Adaptation it in the name of good gameplay, or have a game that's accurate to the source material but, well, not fun. (Incidentally, Pragmatic Adaptation is the right choice; like it or not, you need to turn a profit, and in this case that means selling your soul. Don't want to sell your soul? Then don't adapt someone else's Intellectual Property!)

Props Department
This one is more dependent on the era you choose for your game to be set in, and its technology level. Just take Frickin' Laser Beams, for instance. They depend on their context: if your soldier is wielding the only ray gun in existence, he's important; but if he's only one of millions of clones, all of whom have BlasTech DC-15s, then forget it. Of course, then you can make your hero soldier notable by having him stumble across a sword that glows blue in the presence of orcs. You've got your standard tech level, which all your generic units occupy. Then you have the tech level your characters occupy, which can be either less or more advanced, but should somehow be different.

Casting Director
Most RTS games use voice-acting and CGI, sometimes with Motion Capture and sometimes without, but Command & Conquer uses full-motion video and actually-filmed actors. How you want to do it is up to you, but if you want The Player to take it seriously, you need to make sure your actors take it seriously. The way you do that is to write a good story. Actors are artists like anyone else: they respond, positively, to quality. There's a reason Metal Gear Solid is considered to have some of the best voice-acting in video-game history, even though the job was non-unionized so half the actors had to take pseudonyms to hide their involvement. If you're telling a good story that your actors can believe in, they will do a good job, regardless of the circumstances. Story matters. (If you take only one thing away from this whole dang article, please let it be that.)

Engineering Department
RTS games have fairly unique needs compared to other engines. The functional difference between an FPS engine and an RPG engine are mainly in where the camera is. RTS games by contrast have specific needs:


 * Pathfinding. RTS pathfinding needs to be leagues better than anything in any other kind of game.
 * Numbers of units. For many RTS game types, particularly those emulating Starcraft, you need to handle more units than most engines find comfortable doing. And that's not just about drawing; even if the 300 Zerglings aren't all on screen, they all still need to move around, path to destinations, etc.

So most off-the-shelf game engines aren't going to cut it for you. Some can, but only if you restrict unit counts to smaller than Warcraft III levels. And there are few RTS-specific engines, and virtually none of any real quality.

In short, you're going to have to do quite a bit of grunt work on your own. Even open-source graphics engines like Ogre 3 D probably aren't appropriate, since their scenegraph-heavy design is going to create a lot of overhead in a large-scale RTS game.

Focus on your foundations first. AI, including pathfinding. Optimize this as much as possible. Get pathfinding to work at a particular level. And most important of all, do this before really designing units. Your game design and balancing needs to flow from the basic movement of units. Special movement features (instantly stops in a game where most units take a second to stop, or can move and shoot at the same time) is a balance factor, just like Hp, damage, or anything else. You need to balance the game based on the AI, and therefore you can't go too far in making the game's units until you have your AI mostly locked down.

The Greats
Oh boy. We could list hundreds of hours of addicting gameplay here and still not do a good job. Basically, though, any game you've seen mentioned in the rest of the article is worth playing, even if only for a little while so that you can learn what that game did differently than everyone else. (Sins of a Solar Empire and Rise of Nations, for example, are games you can basically master in 10 hours. You'll probably play longer, because they're fun, but in terms of just getting an understanding of their mechanics, what they do well and what they could improve, it won't take long.)
 * Dune 2 was the Genre Popularizer game, defining RTS as we know it today by combining base management, economy management, force creation and real-time gameplay. Warcraft and Command & Conquer were the first to Follow the Leader successfully and are still names to conjure with today.
 * For games that branch back into the genre's roots as Turn-Based Strategy, check out Age of Empires and Rise of Nations, which is sometimes considered its Spiritual Successor.
 * Finally, Starcraft is the runaway success of the genre, retaining much of its popularity despite being (as of this writing) ten years old, as well as being the national sport of South Korea.

The Epic Fails
And this category could be really big too. Of course, few of them have articles here on the site—primarily because they're epic fails. Ever heard of Nexus the Jupiter Incident? Genesis Rising? Hegemonia Legions Of Iron? Conquest: Frontier Wars? What was wrong with them? Absolutely nothing! Sure, they weren't blockbuster hits, but they weren't bad either. The problem is, the standard for RTS is really high (damn you, Starcraft), and as such even mediocre games just don't cut it. It's like trying to make a Zelda clone: either you go big or you go home.