Illusion is a series of platforming video games licensed by Walt Disney Company and developed/published by Sega exclusively for its consoles Master System, Mega Drive and Game Gear. The series follows the adventures of Disney's cartoon character Mickey Mouse (sometimes with Donald Duck) between various fantasy worlds. The series includes Castle of Illusion, and its sequels Land of Illusion, World of Illusion and Legend of Illusion. The first two games and the last game was released in Master System/Game Gear, and the first game and the third game was released on Mega Drive.
The series take generally place during the middle ages, although in several levels the characters enters into various thematic "illusion" worlds, like the toy and candy worlds.
The series consists in side-scrolling games putting the player in control of Mickey Mouse (or sometimes Donald Duck). The player must defeat enemies and bosses by jumping on them, or collecting items to use as projectiles.
The player can also collect icons to increase health, collect extra lives, or gain bonus points. After defeating a boss, the player is given a magical item (which varies in every game).
In World of Illusion the gameplay changes depending on if the player selects Mickey or Donald, in a one player mode, or if two people want to control both characters at once. For instance, Mickey can squeeze through certain gaps, while Donald must find a different route; if both are playing, Mickey can pull Donald through the gap. Either can stand on the others' shoulders to ascend higher, and lower a rope to pull the other up.
Source: Wikipedia, "Illusion (series)," available under the CC-BY-SA License.