Door

A standard red door.

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 of it.

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 scales on it.

In some stages, the end of the level is illuminated by rays of light. Entering the light will make it brighten until the screen goes back, at which point the regular finish screen appears. If the stage is completed again, the light will be replaced by a golden 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 lead to save points.

In one room in the Great Maze, there is a large purple door which leads to Tabuu's Residence.

Day-changing doors

In The Lake, 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.

There are also day-changing doors in The Great Maze, in an area based loosely on The Forest.

Triple switch doors

In the The Subspace Bomb Factory, there is a locked metal door with three lights on it. In the area, there are three switches throughout the section that look like levers. There is a blue, green, and red switch. Hitting one will make the light of the same color as the switch shine. Once the three switches have been activated, the three lights 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

Hidden doors are doors that are not necessary to enter to clear a level. For example, a door might only appear right after it has been passed by, a switch must be pressed to make the door appear, or it is hidden in an unexpected place, such as where an arrow hints to go down, while the hidden door is actually above.

When a hidden door is entered, it will usually lead into a small room with a Turquoise Cube, and an Orange Cube which is harder to reach. The Orange Cube is also necessary for a higher clear %.

Special doors

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.

Trivia

When a fighter enters a door, they will lose all status effects and items. Olimar will also lose his Pikmin, always having only three Pikmin after entering a door.
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 solo are Midair Stadium, The Ruined Hall, Battleship Halberd Bridge, and The Canyon, as there are no doors in those levels.
There are also two areas in the Subspace Emissary that are entered without doors. In The Ruined Zoo, the player enters what appears to be the entrance to an abandoned aquarium, and in The Lake Shore, they enter a cave through a large opening.