At the north of the plasma gun building, walk onto the first step to the west. Immediately run to the east of the building and drop into the newly-revealed hole to enter the former humans' corridor (sector 458), containing a berserk pack.
In the room with the blue armor southeast of the plasma gun building, open the center portion of the eastern wall to find a stairway leading to the lost souls' cage (sector 427) south of the rocket launcher building, containing a soul sphere and two lost souls.
At the southwest side of the building east of the red key, there is a section of wall (linedef 297) with linedef type 127 (build stairs by 16 map units, fast, switch-activated once) tagged to the nearby, yet to be built staircase, even though it has no switch texture, and a switch with an identical type and tag is found at the very east of the map. This may likely have been a debug switch, meant to test if the stairs would build as intended, that the author forgot to remove before the megawad's release.