Mirror World

A level in a video game which is an altered version of an area you were in earlier. Sometimes you're visiting the same world in different time periods - be they separated by days, months or centuries. Other times, you're visiting another version of the area in an Alternate Universe - maybe even a Bizarro Universe.

Such areas nearly always happen to contain Chaos Architecture, which 'explains' why the Temple of Doom that you visited in the present day has a radically different floor layout ten years in the future.

May overlap with Dark World, might be used for Dual World Gameplay. Not to be confused with Mirror Universe. Also not to be confused with Remixed Level, which is the exact same level with minor differences; this is an "alternate" version of the level, or one with intentional similarities.

Action-Adventure

 * Castlevania: Harmony of Dissonance, somewhat similar to Symphony of the Night, except the player does not visit the second castle after visiting the first, but goes back and forth between the castles throughout the game, becoming aware of it around halfway through..
 * The upside-down castle in Castlevania: Symphony of the Night.
 * Done to great effect in Day of the Tentacle. The game's three protagonists are stuck in the same mansion, but one is in the present, one is 200 years in the past and one is 200 years in the future. A number of puzzles involve taking advantage of various objects that exist in the mansion in more than one time.
 * In the old PC Adventure Game Darkseed and its sequel, you point and click your way through the normal scenes in town, then go through a mirror to visit those places is an H. R. Giger-inspired world.
 * Devil May Cry had a mirror you could pass through. Said Mirror World was horrifying with the bloody color, tilted camera angle and the shifting water effect on everything. Not to mention the random enemies that THROW THEIR EYEBALLS AT YOU.
 * Ghostbusters the Video Game (2009) had this in the Sedgwick Hotel. When you revisit it for chapter 5, it is undergoing extensive remodeling to repair the damage done, ironically, by the Ghostbusters themselves in the first chapter, but this still doesn't explain why some of the hallways on the 12th floor are entirely different now. (The lobby, however, is brilliantly consistent.) Perhaps justified in that the  has been doing some extensive remodeling of her own on floor 12, warping it into a twisted trans-dimensional abomination-world made mostly of cobwebs and corpses wrapped in said cobwebs.
 * Illusion of Gaia for the SNES contains one temple, the Sky Garden, which serves as a variation of this. Basically, you must jump from the "right side up" portion of the garden to the "upside down" portion of the garden, using the various "jump" points to navigate past obstacles in the maze and hitting certain switches that have an effect on the obstacles on the opposite side. No real explanation is given on why, when you jump off a ledge, you don't fall off the edge and plummet to the Earth below (not to mention how you can walk upside down). It doesn't take away from the gameplay or story, though, so don't worry about it.
 * It's Artificial Gravity, of course, which would also be how the Sky Garden itself stays in the air.
 * Legacy of Kain: Defiance often had the two protagonists exploring the same locations while separated by five hundred years. The Spectral Realm may also qualify.
 * The Legend of Zelda:
 * A Link to The Past'' had the Dark World, where a puzzle required you to drain water from one world and use the fact it would fill the other.
 * Majoras Mask has the Stone Tower Temple, which you must flip from right-side up to upside down. Conditions in the "heads" version affect conditions in the "tails" version, and vice-versa.
 * The Past in Oracle of Ages.
 * The future in Ocarina of Time. Certain actions such as planting seeds in the past have effects on the future timeline.
 * Luigis Mansion had the Hidden Mansion, which, in the PAL version only, was a mirrored version of the Normal Mansion made more difficult.
 * Dark Aether in Metroid Prime 2: Echoes.
 * Zack and Wiki have one level in the Ghost House with a mirror, that hides the treasure from you..
 * Zack and Wiki have one level in the Ghost House with a mirror, that hides the treasure from you..

Fighting Games

 * Certain areas of Subspace Emissary in Super Smash Bros. Brawl had you shifting between daytime and sunset of the same level, using it to remove an obstruction -- on both sides -- from one side, that would be indestructible on the other. It probably isn't a coincidence that this game and Kirby Super Star had the same designer.

First Person Shooters

 * In the Marathon Game Mod Marathon EVIL, the penultimate level has a "5d space" portal which leads to a mirror version of the level occupying the same space.

Platformers

 * The four different seasons of Click Clock Wood in Banjo-Kazooie.
 * Utilized in Braid, most Mind Screwingly in the final level.
 * "Zoovania" in the Transylvania world in Bugs & Taz: Time Busters. If you visit during the day, it's a normal zoo. Visit at night and it's changed; most notably all the animals turn into monsters. You can toggle day and night from outside the zoo.
 * Day and night versions of Fungi Forest in Donkey Kong 64.
 * Haven City in Jak III has almost completely been taken over by enemies, but the layout of areas is the same as in the second game..
 * Floria in the Kirby Super Star game Milky Way Wishes had you shifting between four seasons of the same level to get past barriers present during one season but not another.
 * In La-Mulana, almost all parts of the ruins had a "front side" and a "back side." One set of front and back areas in particular is the Twin Labrynths, an area whose front and back side are mirror images of each other.
 * A 3D game based on Phineas and Ferb The Movie: Across the 2nd Dimension has the playable characters going through two dimensions on each level.
 * A major part of Prince of Persia: Warrior Within takes place in a ruined fortress both in the present and the past. You will often need to travel between the times to find away around obstacles.
 * World 6 in Purple is a set of five stages replicating the looks of previous worlds with neons! . Stage 6-1 literally replicates entire stage 1-1.
 * Ratchet and Clank Up Your Arsenal has an actual Mirror cheat. Levels can be played as mirror images of themselves.
 * Sonic CD's Past, Present, Bad Future and Good Future versions of the same levels.
 * A lot of the Special Levels in Super Mario 3D Land
 * Super Mario 64 had Tiny-Huge Island, in which you could enter pipes and travel to the other world. Everything was exactly the same, save for size.
 * Subspace from Super Mario Bros. 2.
 * Super Mario Bros 3 has a level with doorways that lead to a mirror version of that level where the enemies are normal-sized (normally they're huge because the level is in Giant Land)
 * Flopside in Super Paper Mario is very literally a mirror world: you get to it through a mirror, and it's a reversed version of the first town. The people are even the opposite of the people in Flipside. For example, Flipside has a grandmother who gives good advice, which her granddaughter says she is following while doing just the opposite, while Flopside has a grandmother who gives bad advice, which her polite, obedient grand-daughter is willing to follow.

Racing

 * Mario Kart has had Mirror Mode since Mario Kart 64.

Role Playing Games

 * The entire world in Dark Cloud 2. The world you enter by going through the red Time Gates is the same world you're already in, but 100 years in the future. Since so much time passes, the differences between the two can be...striking.
 * Amaterasu Server in Digimon World 3.
 * Dragon Age Origins has a few different examples of this, depending on which origin the player chose. Dwarf nobles, dwarf commoners, mages, and city elves will all revisit their homes about one year after they left and much has happened in the interim, while Dalish elves will revisit the same forest they grew up in, but with a different clan of Dalish after their own clan left.
 * EarthBound has Moonside, which is a twisted, alternate reality version of Fourside.
 * Neverwinter Nights had a temple that you had to enter in the past in order to fix something in the present. You went in in the present, found a way to the past so you could change something, then went back in in the present and your problems were solved.

Survival Horror

 * Eternal Darkness is 12 levels long, which is to say it's four areas set in different time periods, lead by different characters, with different obstacles\items\enemies\routes\etc.
 * Ghost Hunter has two different versions of the DeMontford Mansion. There's a Weird version and an Evil version, so named in the save file titles. There's also a normal version, but only a few rooms of it are visited.
 * In Penumbra: Black Plague, you visit

Non-Video Games Examples

 * The aptly named Mirror World from Kamen Rider Ryuki.