It is guarded by demons (facing away from you), and contains partial invisibility and a berserk pack.

Northeast section: The doors out of this section are one-way you can only enter it from the central region.

This central region is easy to get into, but isn't required to complete the map. All other teleporters return you to the center. The teleporter at the north-west corner (which is glowing slightly) teleports you outside to the north-east chamber. Each time you find a new route to teleport in, you'll open one of the four pillars (which count as a secret).

Central region: The central region is easy to enter, but if you don't know how to exit, you'll find yourself trying random exits.

If you just go for the basic completion, you'll miss out on the many features of the map: The exit should be visible within that room. Take the southern exit, and head to the blue skull key door on the left. The scenic route is possible, but the fastest method is to take the teleport at the south-west corner of the room to reach the north-east room.

Doom skull switch secrets