Key Door
Key Doors (also known as Locked Doors) are doors by which the player cannot enter unless they have collected a Key. Locked doors usually have the appearance of a regular Warp Door with a keyhole.
History
Super Mario series
Super Mario Bros. 2
In Super Mario Bros. 2, if the player picks up a Key for a Key Door, Phanto will chase the player.
Super Mario World
Although Key Doors did not make an appearance in Super Mario World, Keyholes have. They serve the same function and purpose, and they end the level.
Super Mario 64 DS
Keys are found in Super Mario 64 DS. They are used to open Key Doors that hold Mario, Wario, or Luigi.
Super Mario Galaxy
In Super Mario Galaxy, keys can be collected to automatically open locked doors or force-fields. They are found in galaxies such as the Gateway Galaxy and the Ghostly Galaxy. In the Gateway Galaxy and Ghostly Galaxy, a Grand Goomba and a Boo, respectively, each hold a key which is released after Mario or Luigi defeats the enemies.
