Tier 3.

Room 13 rotates, the squareness is an illusion.

A character could go from room 6 to room 14 using rope K; however, the door into room 13 is locked such that most common spells like Knock wont open it.

Stairs L and M go down to Tier 4 of the pyramid.

Stairs G, in room 8, also go down to the next Tier.

While there is rock below these rooms the fog obscures that. So the three stairs going down to Tier 4 are partially encased in fog.

All rooms and stairs are enclosed; however, some cracks and small openings will let the characters see the fog below. When walking down stairs G, L, and M the characters will observe a fog enshrouded passage in front of them.