An underground level. First make the way to reach the bridge, and then get the two keys you will need in order to leave this bloody realm of danger and fear and rise to the surface...
Follow the path from the starting point to the north - you will eventually face a small compartment with a blue-barred switch in it. North of that compartment is a smal isle with a blue torch and some Hell Knights on it. And north (and a step west) of that isle there is a walkthrough wall with a Soul sphere in it.
In the very north of the map in the eastern cage-separated area with the Hell Knights in it there is a door hidden in the north wall (directly noth of the Berserk pack with a Radiation suit and a teleporter in it.