The Door is a Portal entity that opens and closes on input. When closed, it blocks mostly everything. They have a wide variety of applications, such as covering up a portal surface, containing enemies, blocking lasers or light bridges, or blocking off parts of the level, requiring the player to solve a puzzle or reach a button to proceed.
Doors are solid until they are triggered. Even though the door appears to simply move out of the way, the pieces are actually no longer solid. In Mari0 1.6, they have two separate entities: horizontal and vertical. If something is in the way of the door when it closes, they are pushed out of the way. In Mari0 SE, the two entities were merged into one, allowing the user to select the direction from a drop-down menu. If an object is in the way of the door when it closes, it will wait until the object is out of the way before closing. The old behaviour can be restored in the drop-down menu, though.
- It actually uses two graphics, one for the center piece and the other for the main body, which is copied on the top and bottom.
- It's appearance is derived from the doors in both Portal games, featuring the white/blue color/pattern from the Portal 1 door, with the rotating "lock" in the center from the Portal 2 door.