Depending on your starting location, the closest among S-137 and I-137 will have a mana pillar.
There is a mana pillar at Z-142 (can't see on the map because it is blocked by the werewolf)
W-142 and Y-149 teleports to each other.
The gate at A-149 requires the Bronze Key to open, which on a First Playthrough can only be obtained in later stages, making 100% occupation impossible on the first try.
Initially, the large room with two yellow lines drawn through it is only a narrow corridor (between the yellow lines. Also, the enemies with yellow dots don't exist.
Doing the events at both Y-138 and Y-146 (deactivating the trap) will cause the narrow corridor to expand into the full room, and remove the gate at X-142.
Entering the room with magenta outline (entering the trapped room) without doing both above-mentioned events will also cause the narrow corridor to expand into the full room and remove the gate at X-142. Additionally, the yellow-dot monsters will appear. The following turn, werewolves will spawn at the mana pillars at F-140 and F-144.
When replaying this level, the only monsters that spawn are two bats and two earthman.