Bonus Dungeon

""Look, it's the extra dungeon for after you beat the game. Good luck!""

- Alice Margatroid, Subterranean Animism

Where the Bonus Boss usually lives. As mentioned, hardcore gamers (especially roleplaying gamers) often feel cheated that the popularization of video games has lead to a lessening in difficulty.

The Bonus Dungeon will be bigger, badder, and with more levels than the other stages in the game. It will be filled with new monsters. Sometimes game designers cut corners by making the monsters simple recolors of common monsters, but with higher stats.

Sometimes, the Infinity+1 Sword will be waiting at the bottom.

Compare Secret Level. Often a Brutal Bonus Level.

Since these are usually secret levels, expect the examples to spoil accordingly.

Action Adventure Games

 * The Legend of Zelda the Wind Waker has a number of multi-level gauntlets. Only one is required to finish the game, the rest are hidden on islands around the Great Sea. The (optional) final 20 floors after the required gauntlet form the Bonus Dungeon.
 * The Legend of Zelda Twilight Princess has the Cave of Ordeals, a gauntlet of monsters similar to the ones in The Wind Waker, with monsters ranging from a single one of the weakest monsters in the game, to three of the strongest and fastest monsters at the same time. If you can reach the bottom, you'll find an inexhaustible supply of a potion that fills your health gauge and temporarily boosts your offensive capabilities, making you unstoppable in combat... although if you can reach the bottom, you likely won't need that kind of advantage, even against the final boss.
 * In fact, you can go back to it afterwards and find the the difficulty has increased.
 * Also in The Legend of Zelda Twilight Princess, lantern caves. They are huge complexes of tunnels that required your lantern to light the way while you fought your way past monsters and pitfalls, collecting an assortment of goodies on the way.
 * There's an extra dungeon in The Legend of Zelda a Link To T He Past in the ported GBA version. Players can't access the dungeon until they completed the multiplayer Four Swords game. Inside the dungeon is 4 areas with very tough puzzles and color swaps of some of the bosses Link fought previously, along with new behavior patterns. Beating all 4 bosses opened the way to fighting 4 clones of Link from the Four Sword, each Link bearing a different color and abilities that mirror Link's. Beating these bosses only got you statistics of your game data, so it's nothing but bragging rights.
 * The Updated Rerelease of The Legend of Zelda Links Awakening had the Color Dungeon, which was only accessable by playing the game on a Game Boy Color. It included color-based puzzles, such as colored switches and enemies that were only distinguishable by their tunics having to be beat in a certain order. For winning, you got either a Red or Blue Tunic, which put you permanently under the effect of a Piece of Power (increased speed and attacks send enemies flying and do double damage) or a Guardian Acorn (double defense), respectively.
 * The two Oracle games for GBC also included special dungeons, available only in linked games. They could be found wherever you would get the sword in an unlinked game--you start the game with the sword, so you never have to go there.
 * Ocarina of Time had the Gerudo's challenge, out in their outpost, which consisted of solving puzzles in different rooms to collect keys. They keys were used in a maze to get the Ice Arrows. While rumors suggested that the cave could be made Unwinnable, the Fortress is always solvable.
 * Okami features a particularly evil example. In different areas of the overworld there are 3 caves that are home to (slightly) upgraded versions of a previous giant demon spider boss. Defeating them earns a reward, but you can then return to the same cave later to find a demon gate eerily sitting there. Going through forces you to battle wave after wave of superpowered regular enemies. Even the lowliest of Mooks can waste you with a couple of hits in these battles (oh yeah, you have to go through 10 of them to get the reward) and have HP that would make some of the late-game bosses jealous. These battles could be considered a refreshing change of pace compared to the general easiness of Okami if not for their sheer sadism.
 * To emphasize how very cruel some of these are, the most difficult has you face several bosses from the past in groups, usually two or three at a time. Including Don't forget those healing items!
 * The Tower of Druaga, in the Namco Museum Vol. 3 Compilation Release on the Playstation, had two secretly unlockable 60-floor towers both harder than the original: "Another Tower" and "Darkness Tower", the latter having many new treasures. The Famicom and Game Boy versions had a different "Another Tower."

First-Person Shooter

 * Doom 2 from the Doom series had 2 bonus levels "Wolfenstein" and "Grosse" that were Shout Outs to the game Wolfenstein 3D
 * Marathon Infinity has the vidmaster levels. These levels are accessed through a secret area on the last level, and basically put you in three of the hardest levels of the series (One from each game), pitted against the hardest form of each enemy. The reward: Bragging rights.
 * The first two Descent games had several secret levels, where the difficulty jumps from the regular levels' Nintendo Hard to Ninja Gaiden levels. Level 30 (secret level 3) of Descent on Insane difficulty is nearly impossible to beat.
 * Ditto for the second secret level, where to rescue the hostages and get Hundred Percent Completion, you have to shoot the doors from the inside while being assaulted by endless waves of Invisibility Cloaked Hulks and Drillers.
 * Medal of Honor Underground featured a campaign after the main game. Players reprised the role of Jimmy Patterson, hero of the first game, as he stormed the castle of a mad Nazi scientist. The castle was replete with...interesting...new enemy types, including attack dogs operating armored vehicles and automatons made from suits of medieval armor. Patterson even constructs his own automaton - a man-sized nutcracker.

Hack and Slash

 * The Playstation 1 translation of Gauntlet Legends had several of these, including one-level versions of every stage that got cut from the PS 1 version. However, after all the bonus dungeons composed of stages cut from the arcade version, the game had as its final hidden stage... the building the developers made the game in. The sole enemy? A giant hamster, the development team's "mascot."

Platform Games

 * Castlevania has had several Bonus Dungeons over the course of the series.
 * The Towers of Eternity and Evermore in Curse of Darkness - both are fifty floors and feature large amounts of combat, and the latter can only be accessed from the former.
 * The Nest of Evil in Portrait of Ruin. Could only be accessed after exploring 888% (Yes, 888%) of the castle. Mostly just previously fought enemies, with bosses ripped right out of previous games. No real story, though getting to the bottom will net you the most powerful double-team spell in the game.
 * The Battle Arena in Circle of the Moon. Very challenging in that your Magic Meter is drained while inside. (Though a trick can let you cast one spell if you're quick) You can bail out if it proves too tough, but you have to play it completely through in order to get one of the most powerful suits of armour in the game.
 * The Forbidden Area in Aria of Sorrow. It's home to a somewhat good weapon, a powerful piece of armor, and the Claimh Solais, which on top of having a very high Attack stat is remarkably long and swings in an arc, attacking enemies above and ahead.
 * The Floating Catacombs in Symphony of the Night. It's home to Galamoth, the most powerful non-Dracula boss in the game; defeating him nets you the Gas Cloud relic, which makes your mist form harm enemies. Other than the Floating Catacombs are completely optional, since there are no Vlad relics up there.
 * Actually the most difficult boss in the game period; it's extremely difficult to defeat him without game-breaking equipment like the Beryl Circlet and the Crissaegrim, while Dracula himself isn't too hard with normal equipment if your levels are high enough.
 * Actually galamoth can be pretty easy using a fancy dashing technique, Alucard equipment and the sword familiar.
 * In a way, the Stage 5' in Rondo of Blood/Dracula X Chronicles. Accessing it in Rondo requires that you beat the game, and accessing it in DXC requires that you defeat Death, the boss of Stage 5. Until you do so, if you try to get a blue orb (which takes you to lower-path stages) in Stage 4 or 4', Death will change it to red. Oh, and if you think the rest of the game is hard, Stage 5' kicks the difficulty up a few notches.
 * The Training Hall and the Large Cavern in Order of Ecclesia. While the latter is a Monster Arena, the former is strictly a platforming challenge.
 * The "Warp Zones" in Dangerous Dave.
 * Every game in the Klonoa series includes one or more "EX Visions" available after completing the main plot. These are usually much harder than anything else in the game.
 * While not actual dungeons, the Commander Keen series has areas in some levels that don't need to be visited and are deliberately hard. There are even a few levels in the series which can be skipped, and some of them are significantly harder than the rest.
 * Episode 1: Only seven out of sixteen levels actually need to be beaten to finish the game. The remainder, one of which is a secret level, are optional. Getting to the secret level is rather tricky, though.
 * Episode 2: Ten out of sixteen levels need to be finished to beat the game.
 * Episode 3: Sixteen levels again, but only two outside of the final boss have to be finished. Thirteen can be skipped if you can figure out exactly how the teleporter network on the map works. The boss fight at the end without the cheat for God Mode is Nintendo Hard.
 * Keen Dreams: This game is a little different from the other ones in the series. To beat the boss at the end of the game, you need at least twelve Boobus Bombs. As the levels containing them can be completed without collecting them, ID Software put them in six levels of the game for a total of eighteen bombs to find. The absolute minimum number of levels to complete to be able to fight the boss is seven out of eleven before the boss himself. Only one level in the game does not serve as a roadblock or a level to get bombs in.
 * Actually, if you have enough lives, you can die over and over in the same level, and get as many Boobus Bombs as you need in one level. In The Melon Mines, there's an "All-Seeing Eye" (that's what I call it) not far from the entrance which gives you three 1-U Ps and eight Flower Power seeds. It takes a lot of skill to get there, though. So, with effort, you could beat the game while only going through four levels)
 * Episode 4: Eleven out of seventeen levels actually need to be completed to finish the game. One level you don't need to complete is extremely dangerous and the means of accessing it is something of a secret. One level gives you a clue as to how to get to it.
 * Episode 5: Only the secret level can be skipped in this game, and it's only accessible from the second-last level.
 * Episode 6: Of the sixteen levels in the game, ten need to be completed to finish the game. The remainder, including a very dangerous one and a secret level, can be skipped.
 * Eversion has Stage 8, which is unlocked by getting all the gems before finishing Stage 7.
 * Super Mario Galaxy 2 has the Grandmaster Galaxy after completing everything else.
 * Donkey Kong Country 2 and 3 both have Lost Worlds that you need bonus coins to enter, and these coins are hidden in the regular levels.

Role-Playing Games

 * The Ancient Cave in the Lufia series has gone from 12 levels, to 100 levels, to 200 throughout the various games on SNES and Gameboy.
 * Very common with Final Fantasy games, especially in Updated Rereleases and Remakes
 * The Via Infinito in Final Fantasy X 2 was also 100 levels deep, and tied into the plot, having spirits of enemies (some who only died in cutscenes and not in fights with the main character) from Final Fantasy X corrupted into fiends as bosses every 20 levels, finishing off with undead Bare-Fisted Monk Trema.
 * Fanatics' Tower in Final Fantasy VI.
 * Also the Dragon's Den in Final Fantasy VI Advance, which is much more hardcore than the Fanatic's Tower, and has a MUCH harder Bonus Boss.
 * The Soul Shrine ain't no picnic either.
 * The Sealed Temple in Final Fantasy V Advance.
 * There are a ton of these in the original game as well, such as the water tower in Worus Castle (which gave you the Shiva summon), the basement of Castle Bal (for the Odin summon) and several others. Technically, everything after the Pyramid is optional, as you can go straight to the Very Definitely Final Dungeon as soon as you reclaim the Global Airship in the third world.
 * Deep Dungeon in Final Fantasy Tactics.
 * Final Fantasy Tactics a 2 has the Brightmoon Tor. There are three entrances, and each one has the player go through several consecutive battles before facing level 99 opponents on the top. The tor features monsters that only appear there, with insanely high speed stat and incredibly powerful abilities, such as the ability to cast Haste on all of their units, or reduce a target's HP and MP to < 10.
 * The Deep Sea Research Facility in Final Fantasy VIII.
 * The Omega Ruins in Final Fantasy X.
 * Final Fantasy XII has a TON of these, most of them incorporated into areas explored earlier in the game. Among the most difficult: Site 11 of the Lhusu Mines, the top half of the Great Crystal, the Subterra of the Pharos Lighthouse, and Phase 2 of the Henne Mines.
 * Don't forget the Nabreus Deadlands and Necrohol of Nalbudis. How bad is it? The goddamn save/gate crystal tries to kill you.
 * The Gameboy Advance version of Final Fantasy I added four unlockable dungeons containing bosses from the 3rd to 6th games in the series. The PSP added a new dungeon on top of that, called the Labyrinth of Time.
 * Final Fantasy II added the Soul of Rebirth quest for the GBA and the Arcane Labyrinth and Arcane Sanctuary for the PSP.
 * Final Fantasy III had one of these ATTACHED to the Very Definitely Final Dungeon, with a TON of Bonus Boss characters, each guarding a specific class' Infinity+1 Sword.
 * This is actually easier than the rest of the Very Definitely Final Dungeon, and you're specifically told to go through it, though.
 * Final Fantasy IV Advance added two new dungeons, one on Mt. Ordeals, containing new equipment due to the fact this version added the ability to switch party members, and the Lunar Ruins, which contains character specific trials.
 * The DS version lost this ability due to Suddenly Voiced cutscenes, and removed both dungeons. It compensated for this by adding a pair of utterly horrifying Bonus Bosses, Geryon and Proto-Babil, and cranking the difficulty of the game Up to Eleven.
 * Final Fantasy IV the After Years has one at the end of each character's individual chapter, usually given by the Moon Rabbit Challengingway. Golbez gets two.
 * The game Vagrant Story has a bonus dungeon called the Iron Maiden. While areas in the game has a map to show which path leads to where, the Iron Maiden map doesn't. There's minimal to no light in the Iron Maiden, and the enemies are much more menacing than usual, and that's saying something. The boss waiting at the end is the reason why this dungeon is called "Iron Maiden".
 * The Abyss in Wild Arms 1, 3, and Alter Code F. It was smaller in the first game, but all later incarnations had it at 100 levels deep. It exists in 5 as well, along with three other Bonus Dungeons, but it's much smaller.
 * The Chicken Level in Dungeon Siege. Hidden behind a series of riddles and item-gathering quests, this was populated with... well, killer chickens with an extraordinary number of hit points. The level was filled with large amounts of fabulous treasure and hard-to-find items for anyone brave and strong enough to defeat the fowl beasts (pun intended).
 * The Cow Level in Diablo II (and the Hellfire expansion for Diablo), inserted as a response to a rumor from the original Diablo that such a place existed. Later, the 1.11 patch introduced an elaborate Pandemonium quest with several bonus dungeons.
 * Neverland in Resonance of Fate.
 * The Hades Cup in Kingdom Hearts, as well as the Paradox Cups in Kingdom Hearts II. Also, the Kingdom Hearts 2 Final Mix version contains the Cavern Of Remembrance, a bonus level full of very difficult palette-swapped enemies as well as normal enemies with their stats jacked incredibly high--and at the end are no less than thirteen Bonus Bosses.
 * The Netherworld, Auldburg, Trials of Elore, Jewel Beast's Lair, Purgatory, and Shadow Palace from Romancing Sa Ga. You only need to visit one of the first 3 that are mentioned in order to progress the story, You can open up all three before starting the endquests, but
 * Romancing Sa ga 2 had several: The Ice/Snow/Sand Ruins as well as a hidden town which allowed an deeper explanation of the game's backstory.
 * Paper Mario the Thousand Year Door contains a bonus level called the Pit of 100 Trials. This gives you a new badge/item every 10 floors, and Bonetail, the Bonus Boss, lives at the bottom.
 * A similar Pit of 100 Trials is also found in Super Paper Mario.
 * Two of them, in fact, one of which must be completed twice to get everything from it.
 * The Pokémon games have these, starting with Mewtwo's lair, the Cerulean Cave in Pokémon Red and Blue. They are inaccessable until after you've beaten the game, and at the end lies a powerful legendary Pokemon for the player to catch, making it both a Bonus Boss and an Infinity+1 Sword.
 * Pokémon Gold and Silver/Crystal/HeartGold/SoulSilver and Emerald possess a different variant. After the bonus dungeon you encounter, rather than a high-level Pokémon, a trainer with six high-level Pokémon, often the highest in the game. In GSC this is In Emerald it's  Both fights are bragging rights only and give no real reward (although they are in fact repeatable, making them among the best spots to grind high-level Pokémon).
 * The Battle Frontier in the various games can also be counted as a type of Bonus Dungeon -- they are all single player tournaments with various gimmicks, which also tend to be source of the better hold items, evolution trinkets, technical machines, etc. This means that if you are going for Hundred Percent Completion (or wish to be tournament viable) you will need to master these game motes. Unfortunately The Computer Is a Cheating Bastard is in full and blatant effect.
 * The Seraphic Gate in all three Valkyrie Profile games. Silmeria and Covenant scales up the difficulty every time you beat it, and all require them to be beaten at least 10 times to get the Infinity+1 Sword, the latter two being far harder due to reasons stated above.
 * The Star Ocean games all have at least one Bonus Dungeon. The Seven Star Ruins in the first game, Cave of Trials in the second, as well as its Gaiden Game, and Maze of Tribulations in the third, which jacked up the pot by adding Sphere 211, another 100-level dungeon, and the Urza Cave Temple, a more puzzle-oriented Bonus Dungeon. The fourth game brought back the Seven Star Ruins and added the Wandering Dungeon. Many of these dungeons share the same background music (slightly remixed) with the Seraphic Gate from Valkyrie Profile.
 * Yet another 100 floor dungeon exists in Beyond Oasis. There are prizes every 10 levels, and if you can make it all the way to the bottom without having to turn back to restock on supplies, your ultimate prize is an indestructible Omega Sword.
 * Mull's Dungeon in Atelier Iris is only accessible after beating the game and contains a Bonus Boss stronger than the final boss.
 * The Chrysler Building in Parasite Eve.
 * The Moria Gallery from Tales of Phantasia. The later remakes expanded it with even more floors.
 * Completionists playing Tales of Eternia will need to conquer Nereid's labyrinth, which involves five of your characters (including a couple squishy magic users), fighting solo against powerful boss enemies, followed by a difficult battle with the True Big Bad.
 * Tales of Symphonia has Niflheim, a Tome of Eldritch Lore found in Sybak's library. The objective is to dive into the book's underworld and purge the evil from it.
 * Its sequel has two of them, one of which requires you to be on a second playthrough. Bonus doesn't begin to describe it.
 * The Japan-only PS 2-version increases the difficulty of Niflheim further. It adds another five floors, and adds two additional bosses:  Have fun!
 * Tales of Vesperia's Memory Dungeon. The graphics are blurry, it's brown, and all the sound effects sound far away, like you're hearing them on a camcorder recording the actual video game. In here, you fight the party's memories, and with that, every enemy they've faced in the game. This makes for some weird situations, like Stone Wall White Mage vs.  and Grumpy Old Man vs.  Strangely, for a game whose characters lampshade many things such as CrackPairings and Dude Looks Like a Lady, this wasn't remarked about at all.
 * The PS 3 Updated Rerelease ups the ante with the Garden of Izayoi, an incredibly long dungeon with the gimmick of progressing through the floors by way of actual combat; once you defeat a group of enemies, paths on the battlefield open up for you to traverse to another battlefield with more enemies, and you make your way through several floors of mazes. There are plenty of new Bonus Bosses, including a horrific "monster" called, which appears to have taken the title of "most difficult boss in the Tales Series."
 * The 60-floor bonus dungeon of Tales of Destiny is a remake of The Tower of Druaga. A 10-floor version of the tower is the bonus dungeon in Tales of the World: Narikiri Dungeon 3.
 * Monad block in Persona 3. The game's ultimate boss can be fought on the final floor.
 * Persona 3 Portable has the Vision Quest, hosted by in the Desert Of Doors from FES. While not a dungeon in the same sense as Monad, it does feature Bonus Boss versions of all the Full Moon Shadows as well as hidden boss that some are claiming is harder than . Yes, you get to fight
 * Pork City in The World Ends With You.
 * Chapter 8 in Dark Cloud 2, which comes after defeating the main villain. Long story short, you go through an extra dungeon and end up fighting a hidden boss.
 * Shin Megami Tensei Nocturne has both the Labyrinth of Amala and the Bandou Shrine. Completing the labyrinth gives you a sixth ending (and Bonus Boss), and the shrine gives you the chance to acquire the secret 25th Magatama.
 * Crossbone Isle of the first Golden Sun. Not as difficult as a good deal of the examples listed already, but still can be a challenge.
 * The second Golden Sun had Anemos Sanctum, as well.
 * And Treasure Isle, Yampi Desert Cave, and the turtle's secret island, all of which had their own bonus bosses (which did not necessarily have to be beaten before taking on the Anemos Sanctum, but you might as well since you had to at least progress part of the way through each to get the Djinn inside before you could unlock it.)
 * Grandia Xtreme's Vortex Corridor.
 * The original Grandia also contained no less than three bonus dungeon; the Castle of Dreams, the Soldiers Grave, and the Tower of Temptation, with the former two being available only for a limited time, and the latter being available to near the end of the game but nigh impossible to actually find. All of them have significantly ramped-up enemy difficulty (but absolutely abysmal experience payout), and all of them contain some of the most useful equipment for that point of the game.
 * And Grandia II has the Raul Hills labyrinth, which hide the best defensive/recovery mana egg in the game.
 * After beating the final boss in Digimon World, there is a Bonus Dungeon that has no set location. The entrance is in one of many dungeon entrances around the map. Inside this Bonus Dungeon are color swaps of generic enemies that are extra powerful and at the end is the final boss once again, only this time at the highest health physically possible.
 * The most recent Dragon Quest games have added bonus opportunities as well.
 * Dragon Quest VI allowed you to defeat the equivalent of Satan, upon which he beats up the normal final boss for you.
 * Dragon Quest VII features a Bonus Dungeon where you fight God. Literally.
 * Dragon Quest VIII's Bonus Dungeon gives us the Hero's backstory.
 * The PSX/DS remake of Dragon Quest IV includes a bonus dungeon which expands on the story, even allowing you to redeem the (apparent) Big Bad, and the former final boss!
 * The PS 2/DS remake of Dragon Quest V includes a bonus dungeon unlocked after beating the main game. The final boss of Dragon Quest IV lies at the end, with the difficulty significantly ramped up. Beating him unlocks the last T'n'T board and beating that nets you the last two recruitable mons, who at this point are just for bragging rights. The real challenge is beating the Bonus Boss in under fifteen rounds, which earns the final Knick Knack for your museum.
 * Baldurs Gate: Tales of the Sword Coast contains Durlag's Tower, a looming castle crammed full of thoroughly unpleasant enemies - and very large traps.
 * Baldurs Gate II: Throne of Bhaal gives us Watcher's Keep, a five-story dungeon (plus one extra for the boss fight) featuring some of the most complex puzzles and challenging fights in the game, eventually climaxing in a fight with, who is not only, as a good Bonus Boss should be, the most poweful enemy in the game, but
 * Of course, some players created game mods making him harder just to ramp up the challenge further. Some people...
 * Which are nothing compared to the mods making the final bosses (of both the original and the expansion) easily 10 times harder to fight. And add a Boss Rush, because just fighting them one at a time is for sissies.
 * The Updated Rerelease of Chrono Trigger adds Dimensional Vortexes unlocked after the final boss, as well as the Lost Sanctum.
 * Even the original SNES version of the game had the Black Omen, an optional dungeon (although necessary to access New Game Plus) that can, through the magic of Time Travel, be cleared three times for maximum loot.
 * .hack gives us the Bonus Dungeons after the end of every game. In G.U. one of those is called the Forest of Pain. How utterly appropriate.
 * Pokémon Mystery Dungeon loves these. Of particular note is Purity Forest from the original pair of games. You can only bring one Pokémon in, which is brought down to level one. Also, all your items and money not in storage are destroyed. Good luck.
 * Similar to Purity Forest is Zero Isle in the second pair, which is divided up into four parts. Zero Isle North simply doesn't give you any EXP, but South, East, and West drop you down to level one at the start, you can't bring items to Zero Isle South or West and can only bring 16 items to Zero Isle East, and Zero Isle West also limits you to just the one Pokemon!
 * And let?s not forget Destiny Tower, in which you can only enter with one Pokémon, which is dropped to level one, enter with no items or money, all IQ skills nullified, hidden traps remaining hidden, and the inability to be rescued if you faint!
 * Probably about a third of Xenosaga 2 was side-quests and another third Bonus Dungeons.
 * Shiren the Wanderer has a bunch of extra dungeons you can take on after beating the main game, including the Kitchen God Dungeon (a special dungeon where you start with Bufu's Cleaver, a weapon that can turn enemies you kill with it into meat), the Cave Behind the Scroll (a possibly shorter dungeon where you start with a Trap Armband, which enables you to pick up and place traps and use them against enemies, as well as gain experience for killing them with traps), Fay's Final Puzzle (a 99-floor marathon where even herbs and scrolls that you find will be unidentified), the Tainted Path (another 99-floor dungeon, with very strong monsters and a boss at the end), the Ravine of the Dead (a 50-floor frolick with tougher monsters, fake stairs, and lots of Monster Houses), and the Ceremonial Cave (a 30-floor labyrinth with tough monsters and another boss). The first three of these dungeons don't allow you to bring any items or money, and you can't bring companions into Fay's Final Puzzle.
 * The "Another Goddess" quest in Half Minute Hero, aside from being very long for the game's scope (most levels last about 30 seconds, while this one will take a good five minutes), harkens to another Marvelous Entertainment-created RPG: Valhalla Knights (the title even changes to reflect this). It's accessible during normal play, but because of the major change in style, the Time Goddess urges you to walk past it, on to the next quest. In order to actually play it, you have to defeat 108 bosses first. Harsh.
 * There's six of the things in Last Scenario, seven if you count the one that's really just a sequence of four bosses. Luckily, all of them give you some very nice rewards.
 * Blue Dragon has the downloadable Shuffle Dungeon, which gives you several new items to collect and some new monsters to fight.
 * Torchlight has the Shadow Vault, known in Fan Speak as the Infinite Dungeon.
 * The Megaman Battle Network series is known for its expansive bonus dungeons, taking the form of huge mazes with doors that must be unlocked by completing some objective elsewhere in the game. BN2 had, 3 had Secret Area, 4 had Murkland, 5 had Nebula Area, and 6 had the Graveyard, a Super Mode Boss Rush.
 * An early example in Sword of Vermilion. Unlike all other dungeons in the game, nobody ever asks you to visit, or even mentions the existence of the dungeon where the Death Sword is found.
 * Phantasy Star IV had a couple of optional dungeons that contained some nice loot, and in one case a Continuity Nod to the previous installment.
 * The Server Room in Opoona. It opens up about midway through the game, but actually challenging it at such a point is not especially advisable. In addition to containing Mooks that are extremely fast, can heal themselves, and prevent you from using your Force (magic), the battle stages are full of bombs, which prevent you from using just about any hit-all abilities lest they explode. (And if they do so, they'll knock off about 100 HP--about three or four is enough for a Total Party Kill.) And if that doesn't kill you, the room is also home to Salamanders, one of the game's most brutal Boss in Mook Clothing monsters. However, you can leae at any time to save and heal without losing your progress.
 * The Fallout New Vegas DLC Lonesome Road adds the Long 15 and Dry Wells maps, which you may or may not have  previous to their unlocking.
 * Wasteland, the original Post-Apocalyptic RPG, had this in the form of Finster's Head. A one-man-solo "dungeon" in a party-oriented game that comes right after what passes for the game's Wham Episode can catch you by surprise with its (entirely optional) Bonus Boss that yields the largest XP boon in the whole game (DOUBLE that if you kill him in melee) and an inventive puzzle maze.

Shoot Em Ups

 * The Touhou series, unusually for Shoot'Em Up games, has pretty much made a tradition out of this. Perfect Cherry Blossom went one step further by having two Bonus Dungeons.
 * Alice and Marisa's Wrong Genre Savvy in Subterranean Animism leads to the page quote in that game's extra stage.

Survival Horror

 * The Suffering features a pseudo-bonus dungeon in Chapter 19. If you deviate from the path that the AI leads you on, you can find a cave with a few NPC's and a slew of overly powerful monsters. The reward? The final component to the super-secret Flamethrower weapon (which certain exploratory players will have discovered several chapters prior).

Turn-Based Strategy

 * The Hellgate from Tactics Ogre was 100 levels deep, and interestingly actually tied into the plot, as the bottom level was where one of the villains in the game had retreated to. Beating him didn't change the main plot of the game, though. In order to get the ultimate "bragging" item in the game, one had to go through the Hellgate twice, as well as get 4 specific weapons from special encounters with recolored monsters.
 * Beauty Castle and the Alternate Hell from Disgaea Hour of Darkness, as well as a world within every item which is generated randomly. Fittingly, the Alternate Hell was the Bonus Dungeon for the previous game, La Pucelle Tactics. There is a similar version in Phantom Brave, which is yet another in the Nippon Ichi line of Turn Based Strategy games.
 * Also fitting in that the Beauty Castle is the last dungeon in another Nippon Ichi game, Rhapsody: A Musical Adventure.
 * Fire Emblem: The Sacred Stones has two of these: the Tower of Valni, and, more fitting, as it is only available toward the end of the main game, the Lagdou Ruins.

Wide Open Sandbox

 * After trudging through Zero's first two missions, which involve shooting down/fighting with toys on a very tight timer in Grand Theft Auto San Andreas, you are treated to...an RTS mission. However, it is probably the most fun mission in the game since it's virtually impossible to screw up, and hearing David Cross cheer you on when you do well at it creates quite the fuzzy feeling. Oh, and one of the previous scrappy levels becomes infinitely replayable after you beat it, although there is now no longer a penalty for failing it.