Netstorm

A Real Time Strategy blended with a little Tower Defence to create a smoothie of intriguing possibilities and a whole lot of headaches. Combat units are stationary buildings, that you cannot control once built.

The landscape is scattered islands in the air, which makes up the (unimaginatively named) world of Nimbus. You control (or possibly are) a high priest who worships the three furies of Nimbus (Thunder, Rain, Wind) and your battles somehow relate to their unending battle for dominance below the clowds. You build a temple to the furies, they give you a variety of towers to blow up the other guys towers and temples. The aim of the game ? Capture enemy priests and ceremonially toss them off the islands to be eaten by the furies.

Getting from one island to another is done by means of bridges that your temple makes. You get given small sections which you join together to make large bridges. First off, you need to go and gather resources which in Nimbus is Storm Power. This appears from geyers, most of which are tiny little islands spread around the map. You bridge to them and send your mooks off to collect the crystals. Secondly, you need bridges to get to the other guys island and blow his stuff up. You can place units on your island but mostly that will put you out of range of the other guy, and the only other place you can build units is attached your own bridges, that in turn connect to your island. Bridges are very hard to destroy, and correctly placing them is actually one of the major skills of the game. Particularly, placing bridges very quickly and in such a way as to prevent an enemy from placing his own bridges and units is pretty challenging, and doing it fast and so you can still attack from them is an art form.

This brings us neatly on to the units themselves. Each of the furies has its own tech tree, and there is a general purpose one (Sun) that any side can use to fill in the gaps in its own tree. Each tree you want to build units from needs a specific workshop built, which costs a fair amount. For any given battle, players normally stick to at most one tech tree plus the sun tech tree. Each tech tree has its own strengths and weaknesses, but in principal contains: A cannon-type unit (that shoots stuff), a blocker (blocks shooty things), barrier generators, an anti-air defense tower , an air-attack unit (you build a base unit that continually builds little attackers) and ground and air transports for capturing storm power. In no particular order, the tech trees are:

Thunder - Lots of damage, but not too flexible.

The Thunder Cannon does the most damage in the game, and has the longest range, but must be built facing one way and cannot turn once its placed.
Thunder's blocker unit, the Bulwark, has the most hp of anything and is invulnerable to air attacks, but still takes damage from ground shooters normally.
Thunder's anti-air unit is the Vandor Tower, which shoots out lightning in a small radius around it.
The Barrier unit is the Arc Spire: You build two of them on a horizontal or vertical line, and lightning begins to course between them, killing or at least heavily damaging anything that tries to cross the barrier.
Thunder doesn't have an air attacker.
The thunder transport is the Bulf which is a large woolly mamoth type creature that can go collect things for you. It's very slow but also the toughest transport.

Wind - Low damage but very flexible.

Wind's cannon is the Crossbow which has a comparatively short range, but is also a very flexible cannon. It doesn't turn, but can hit anything inside a ninety-degree arc in front of it. It also does double duty as an anti-air cannon.
The Wind Tower is very weak, but on one side it is invulnerable to all cannon fire. Obviously this is quite a plus.
Wind's air attack unit is the Dust Devil, which is invulnerable and does a lot of damage but doesn't last particularly long.
Wind's ground transport is The Sail Skeet, which is a essentially a small sail-boat on wheels, which is the fastest transport, and it's air transport is the Airship, which is slow but also quite tough.

Rain - Decent damage and flexibility.

Rain's cannon is the Ice Cannon. The same as the thunder cannon, it can only fire in one direction but it does less damage and has less range. However, it is also cheaper, and it has a small blast radius where it hits. It also doesn't fire continuously, but in bursts with a break in between.
Rain's blocker is the Ice Tower, which has fairly low hp, but it also re-grows once it's destroyed, and in fact can't be fully destroyed. Numerous layers of them can provide a permanent barrier to shooty things.
Rain's barrier generator, which doubles as an anti-air unit, the Acid Barrier. You set up two towers on a direct line, and this sets up some kind of force field between the two. If any flying units try to cross the barrier, they get immediately slurped up and killed. This is obviously pretty useful, but you need to take care, because the towers themselves are not protected.
Rain's air attacker is the Man o war. A flying jellyfish like creature that shoots lighting, with very long range and which does good damage, but which is also slow moving.
Rain's ground transport is the Crystal Crab, which is pretty fast, reasonably tough, and will attack other transports and steal what they are carrying.
Rain's air transport is the Cloud Floater, which is reasonably fast and quite hard to kill, as most shots will pass right through it.

Sun - Nothing excels, but cheap and patches up the other trees.

The Sun Cannon is basic, but useful. It is the only pure cannon that can turn after you place it and can fire straight up, down, left and right. It has decent range and hitting power although nothing special. It is cheap and effective.
Sun's blocker is the Stone Tower. Mediocre and with no special features, it just gets in the way.
Sun's anti-air is the Sun Disc Thrower, which flings large bronze discs within a short radius. Unlike the other AA units, the sun disc thrower will attack ground units as well, and it's unique 360 degree fire arc can make it useful under lots of situations.
Sun's air attacker is the Whirlygig. Again, no particular strengths, and the flyers themselves are very weak, but they get the job done.
The Sun also has a unique barrier generator called the Sun Barricade. You build two towers, and there is a force field between them. In this case, this is a real barrier, that defends against shooty things and lasts forever.
Sun's ground transport is the Golem and it's air transport is the Balloon. Both are weak and slow, but cheaper than their counterparts of the other Furies.

So yeah, you build between islands, you shoot units with your units, you blow the other guy's temple up, knock their priest out, capture him and tear out his soul.

Although utterly unrelated to game play, this is a game with a lot of flavor, and for a world that you never really get to know, it takes quite a lot of pains to get you interested. Everything has an encyclopedia entry, and there is a whole lot of quotes and really good flavorful writing. It's actually kind of sad that no other games in this universe were made. We also get a glimpse of the normal people living in this horrible, war torn world, living in sunken houses and clinging to the edges of the islands. It's pretty cool.

Also, for some years now, Netstorm has been freeware and you can get it from the great guys at NetstormHQ. This troper strongly recommends you give it a go. It's a great, if hideously frustrating game and it will absolutely test your strategy skills to the limit.

Tropes used in Netstorm include:

Beating a Dead Player: You don't win just by blowing up all the other guys stuff, you need to shoot HIM a bunch and then sacrifice him to your gods.
Check Point Starvation: No check points, no saves of any kind once you get into the fighting. In the harder missions, this seriously ratchets up the difficulty. Particularly since bridge pieces are random this can effect how quickly you can get to resources or how you place towers, and that means that starting over because you screwed up later on can be tough just to get back where you were.
Color-Coded Armies: Not a vast amount, but there are colours that tell your sides appart. Your islands and towers have an appropriate border, while your transports and priest are the right colour. You are blue, the bad guys are variously red, purple, yellow and green.
Com Mons: You start out just with the sun cannon and sun disk thrower, but as soon as you get ahold of the shiny toys they are if not forgotten, they are at least relegated to marginal use. Every element has a better cannon and a better anti-air choice, although they do cost more.
Cruelty Is the Only Option: Can't win without tearing out the bad guys souls. But no-one seems to have that much a problem with that but still...
Elemental Rock-Paper-Scissors: In theory, Wind beats Thunder (Thunder Cannons can only shoot in one of the 4 cardinal directions, and Wind Towers are invulnerable on one side), Thunder beats Rain (Ice Towers are weak but regenerate, Thunder Cannons can destroy them very quickly, and Bulwarks are invulnerable to air attacks, such as Rain's powerful Man-o-wars), Rain beats Wind (Ice Towers are never permanently destroyed, so you can use them as permanent blockades over which the short-ranged Crossbows can't shoot far. Also, Ice Cannon shots ricochet off buildings they hit, allowing them to hit Wind Towers in one of the vulnerable sides), and Sun can't take Wind, Rain or Thunder in a direct fight, but has ways to attack from safety (for example, Sun Disc Throwers can attack diagonally, unlike every cannon, and Sun Barricades, while weak themselves, create a barrier between them impervious to cannon fire).
Excuse Plot: Oh yes. The plot of the campaign is kind of non-existent. You get told that you are part of this big war thing, but the characters and sides aren't in any way expanded upon. You basically go and fight people. Sometimes you have to fight a few at a time because you were betrayed or something, but no-one is really paying attention.
Fire, Ice, Lightning: There's four building types with different attacks. Sun is well-rounded, Air performs quick and fast attacks, Rain tends to be persistant attacks, and Thunder tries to overpower with raw strength.
Immune to Bullets: Forcefields and a few defences are invulnerable to projectile attack.
Inverse Law of Utility and Lethality: The more powerful a tower is, the more it will cost and the lower range of movement it will have. Most obviously, the thunder cannon and ice cannon have powerful long ranged attacks, but you better hope that you pointed it at something that's really worth killing, because it won't be shooting anything else. The crossbow and sun disk thrower can shoot across a much wider area, but at much less range. Oh and they can shoot aircraft too. The sun cannon is somewhere in the middle.
Last Chance Hit Point: Slightly. If your priest is incapacitated (by being shot repeatedly in the head with a gun that shoots things about his size for example) he doesn't die, he's just in a coma or something and has a magic coma shield. If you still have a temple you will re-charge health and assuming you aren't still being shot eventually return to health.
Level Editor: A whole bunch of fan made maps and even campaigns and story missions have been made with such a tool.
Mirror Match: Most of the time you have a different tech tree to your enemies (so you can rock/paper/scissors him to death) but sometimes you have to beat the same tree. Also, all the priests are just pallet swaps, so in a sense every fight is a mirror match.
Overheating: The ice cannon shuts down briefly between bursts before re-starting.
Recycled Soundtrack from Shattered Steel.
Regenerating Health: The ice tower that constantly re-grows, and the priest can sometimes re-gain health too, but nothing else in the game can without specific spells.
Reinventing the Wheel: In multiplayer, obtaining all techs and winning another game allows you to advance to the next rank for combat bonuses. The catch is that you need to re-research all of the units.
Shattered World / Floating Continent: Obviously, what with the floating islands. Apparently the islands have some sort of propulsion system that is only used off camera, but you can never use when it would actually be useful.