A door is the one place the board changes permanently. Which also makes it the one irreversible decision in the catalogue.
Behaviour
An edge tagged door is impassable in both directions until the matching key
is collected. Once you have the key it is yours for the rest of the level; the
door opens on first passage and stays open.
A closed door blocks not only movement but sight: a line of sight stops at a closed door edge. Open the door and chains start running through it.
You can of course undo the move; what you cannot do is close the door again while moving forward through the level.
How you read it
The door art matches the colour of its key and carries a symbol alongside the colour — information is never given by colour alone. Locked, the door is drawn shut; once opened it stays open, so you can look at the board and count which doors you have already been through.
Taught in
Taught in1-8
A strong tutorial level: remove the rule from the data and the shortest path drops from 13 moves to 7. Going for the key is a six-move detour and there is no negotiating it. The player understands what the key is for without experimenting, because the door plainly cuts the road.
Why blocking sight matters
In act 3 the same door serves as both wall and passage: shut, it holds back the gaze of the sentry behind it; open, it becomes a route but releases that gaze too. When to use the key becomes a question as important as the level itself.