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.

Doors act as a checkpoint in the Subspace Emissary. When the player runs out of stocks and continues, he will appear next to the door that the player had last entered.

Doors also help increase your clear %. You must enter all the doors in all the levels.

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 silver in color, and slide open instead of opening from the middle. In the stages which feature the Research Facility and the Subspace Bomb Factory, the doors are gray in color with black stripes.

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 fighter/boss behind the door.

Teleport doors
In The Lake stage, there are some doors that are yellow or purple, 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, e.g. Some areas are only accessible by entering one door. The members of the Subspace Army are different as well. What a player does in one world affects what happens in the other.

There are also teleport doors in the Great Maze.

Hidden doors
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 you pass it, you have to press a switch (also hidden) to make the door appear, etc. Or they are hidden in places where you least expect, e.g. an arrow tells you to go down, while the hidden door is on the top. But there are usually some clues to help you find them.

When you enter a hidden door, you will usually be in a small room with a turquoise cube near you, and an orange cube which is harder to reach. The orange cube is also necessary for your clear %.

Trivia

 * When a fighter enters a door, he/she will lose all status (e.g. metal, giant, invincibility, etc.) and items, including Trophy Stands.