The door, closed. Binder clips on outside right at floor clamp the "hinged" side to the longer wall. One binder clip at the upper left angled to clip door to wall. This doesn't clip the bottom left corner in any way, so I place a heavy object outside and against the wall, so that from the inside the door doesn't "give" (much). My 3 boy rats have never tried escaping from this door. My older girls of long ago, in a different rat room, tried relentlessly to escape. I think if they had been in this rat room, this version of a room would not have held them.