The Tree Door is the only door that blocks access vertically, more in the manner of a trapdoor. The passage is blocked by wooden planks, which slide open when the door is opened.
It takes up 4.33% capacity in design mode, and has many variable settings that it shares with the other doors. It can start locked (in which case it with require a key) or unlocked, and will open when the player either comes into contact with it or unlocks it. The door can be set to take up to five seconds to open and close, or to not close at all. It can be set to lock upon closing.