Warp Door

Warp Doors are doors appearing in several Mario franchise that take the player to another part of the level. Most of them are unlocked, while some, known as Key Doors, require a key to be opened.

Super Mario Bros.: The Lost Levels
In Super Mario Bros.: The Lost Levels, Warp Doors appear only in two castle levels (from Worlds 8 and D, respectively), after Bowser is defeated. Both lead to Princess Peach.

Super Mario Bros. 2
As Warp Pipes were absent in Super Mario Bros. 2, Warp Doors were the main means of transporting between areas. The game consisted of three different types of Warp Doors: red swing-open ones, doorless entryways, and special doors with keyholes that required the players to search for the Key in Phantos' lairs to open those doors. In Super Mario All-Stars and Super Mario Advance, a pair of dark yellow double doors (similar to Super Mario World's) replaced doorless entryways while in the overworld areas, and a giant iron bar door with a Bob-omb emblem appeared in World 6-3.

Super Mario Bros. 3
Doorless entryways were also used in Super Mario Bros. 3, despite Warp Pipes returning, until Super Mario All-Stars and Super Mario Advance 4: Super Mario Bros. 3, where they were replaced with Warp Doors. Big Warp Doors also appear in both of Bowser's boss arenas, which open after he is defeated, and both lead to Princess Peach. Another variant known as Magic Doors appear in World 4-6; when entered, Mario can transition between giant and normal-sized versions of the stage.

Super Mario World
Warp Doors are used predominantly in Ghost Houses, Fortresses, and Castles in Super Mario World, to transport Mario or Luigi to another room. Big ones debut in this game, serving as boss rooms' entry points.

New Super Mario Bros. series
In the New Super Mario Bros. series, Warp Doors have appeared in many Ghost Houses, Castles and Towers, as well as at the end of each Castle and Tower throughout the series leading to the boss.

Super Mario 3D Land / Super Mario 3D World
Warp Doors also appear in Super Mario 3D Land and Super Mario 3D World, in Toad Houses and in some stages.

Super Mario Maker
In Super Mario Maker, Warp Doors can be used as course elements. They transport the player to a different part of the area. Up to 8 doors (4 pairs of doors per area) can be placed in the level. As of the 1.20 update (November 2015), players can knock multiple times on the door in the Course Maker to reveal Weird Mario. The 1.30 update introduces a variation: The P Warp Door, which cannot be activated until pressing a P Switch. Another variation introduced on the 1.40 update (March 2016) is the Key Door; they can only be opened with a key, another feature introduced in this update.