This pillar is the doorway. It works out the point in the middle of the arch, 13 from itself along the wall, and keeps it in its own +$09 to +$0B -- the step fields, which a pillar never needs, borrowed because IS_NEAR_TO compares +$09 to +$0B of one record with +$01 to +$03 of another. Then HANDLE_EXIT_SCREEN lets the player out of the room if he is in the doorway and past the wall, and ARCH_NUDGE_TO_CENTRE nudges him onto the arch's middle line if he is near it.

An arch in a wall of constant U (not mirrored) has its other pillar 13 further along V, and its doorway reaches 15 either way in U and 6 in V; one in a wall of constant V (mirrored) has its other pillar 13 back along U, and a doorway 6 in U and 15 in V.

A pillar's +$08 is not the room it stands in: the builder (BUILD_ROOM) gives every piece of a scenery entry the byte after the entry, which for a doorway is the room it leads to.