Doors are found in the Subspace Emissary to teleport the player into another room. Doors are usually red in color with a pointy top, with blue light shining out from the edges.
Doors act as a checkpoint in the Subspace Emissary. When the player runs out of stocks(i.e. game over) and continues, he/she will appear next to the door that the player had last entered.
Doors also help increase the player's clear percentage. All the doors in all the levels must be entered.
Types of doors
While the doors are usually red, there are different types of doors as well. In the stages in the Halberd, the doors are bronze in color, and slide open instead of opening from the middle. In the stages which feature the Research Facility and the Subspace Bomb Factory (i.e. the Isle of the Ancients), the doors are gray in color with black stripes. In the Subspace stages, the doors will open like the red ones but are purple and have what appear to be dragon scales on it.
In some stages, the end of the level has a gold, shiny door.
In the Great Maze, doors with black smoke emerging from the edges indicate that there will be a Subspace Clone/Boss behind the door. Also in the Great Maze, there are orange doors with a yellow circle in the middle which leads to save point.
In one room in the Great Maze, there is a large purple door which leads to Tabuu's Residence.
In The Lake stage, there are some doors, namely day-changing doors that are yellow or sky blue, with a triangle in the middle of it. When a player enters that kind of door, they will be teleported into a similar world, but are different in some minor ways, but the most noticeable is the background. One of the areas is during the day, and the other is during the evening. The members of the Subspace Army are different as well. What a player does in one world affects what happens in the other. It is likely that going through one of these doors will transport you forward and backward in time, depending on the color.
Triple switch doors
In the The Subspace Bomb Factory, there is a locked metal door with three holes (for the jewels to open the door-a red hole for a red ruby, a green hole for a green emerald, and a blue hole for a blue diamond) on it. In the area, there are three gems throughout the section that look like keys. There is a blue diamond, a green emerald, and a red ruby. Hitting one will make the jewel of the same color as the switch shine. Once the three gems are in the proper jewel holes the door to the next room will be activated, the three gems at the door, as well as the edge of the door will shine and the player will be able to pass.
There is also a triple switch door in the Great Maze.
Hidden doors are doors that are not necessary to enter to clear a level. They are hidden in some special ways, e.g. It only appears right after it has been passed by, a switch must have been pressed to make the door appear, etc. Or they are hidden in places where it would be least expected, e.g. an arrow hinting to go down, while the hidden door is actually above, but there are usually some clues as to where they are.
There are three special doors which only appear after the Subspace Emissary has been completed. Those three doors can be found in The Forest, The Swamp, and the The Ruins. When they are entered, there will be three challengers that have not appeared in the Story Mode. They are Jigglypuff, Toon Link, and Wolf. Defeating the characters will make them join the party.
- When a fighter enters a door, they will lose all status effects and items. Losing items also makes obtaining boss trophies much harder, as it would be much easier to get a Trophy Stand in a regular level. Olimar will also lose his Pikmin, always having only three Pikmin after entering a door. Also, any character that can charge their weapon and store the charge for later will lose the charge.
- When a fighter enters a door on the ground, they will walk into it. However, when fighters enter a door that is in midair, they will tumble toward it. This animation is the same as being Footstool Jumped off of.
- Through hacks, it is possible to play as Giga Bowser in the Subspace Emissary, but he is not able to open doors, making most levels impossible to complete with player 1 as Giga Bowser. The only four levels that Giga Bowser can complete are Midair Stadium, The Ruined Hall, Battleship Halberd Bridge, and The Canyon, as there are no doors in those levels.
- In The Subspace Bomb Factory (Part II) level in Adventure Mode, the jewels to open the locked metal door look similar to the 7 crystal stars to open the thousand year door in Paper Mario: The Thousand Year Door. The seven stars are: the Diamond Star, the Emerald Star, the Gold Star, the Ruby Star, the Sapphire Star, the Garnet Star, and finally- the Crystal Star. Once Paper Mario has all the Crystal Stars- the Diamond Star to the Crystal Star the Thousand Year Door will be unlocked. However, the room where the next room is guarded with the locked metal door (almost like a magic door making this place look like a treasure trove) the crystal stars from Paper Mario: The Thousand Year Door are replaced with the gems to unlock the locked metal door. And once you have all the jewels in the door in the proper gem holes- the red ruby in the red hole, the green emerald in the green hole, and the blue diamond in the blue hole; the door opens.