At the northwest hallway, one of the southern walls to the north is aligned differently from the rest. Open it to find a teleporter leading to security armor at the south of the level. (sector 5) Walk off either side of the platform to return.
In the final hallway before the exit, a candle sits in front of the northern wall. Press on the wall behind it to find a box of shells. (sector 122)
In cooperative multiplayer mode, Player 2 starts the level (and respawns) standing close to and facing a wall.
In the outdoors area outside of the northeast room, four linedefs (556, 651, 781, and 765) on either side of the northeast and southeast windows are double-sided without a back side, producing the Hall of Mirrors effect. However, these cannot be seen unless you activate the noclip cheat and walk outside through the windows.