Reached through UPDATES. This pillar is the doorway. It works out the point in the middle of the doorway, 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 CHK_PLYR_NEAR_ARCH marks the robot's legs as in the doorway if they are near enough, which lets them past the line of the wall (HANDLE_EXIT_SCREEN), and ARCH_NUDGE_TO_CENTRE nudges him onto the doorway's middle line if he is near it and facing through.

A doorway in a wall of constant U (not mirrored) has its other pillar 13 further along V, and 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 reaches 6 in U and 15 in V. The backgrounds put the middle of every doorway at 128 along its wall.

As Knight Lore's adj_2_4_hflip and the routines after it, and Pentagram's FIRST_PILLAR. The nudge differs from both: see NUDGE_ALONG_V.