A small city-like area with three black buildings, each of those containing one key you will need - but all are heavily guarded. Get the weapons in the right sequence and be careful not to fall victim to the traps that await you. You will only need two keys to finish the level, but I recommend getting all three for maximum fun.
In the building in the southwest in the first room ride the lift up to the ledge from where the chaingunners attacked. From there you can easily jump into the central circular pond - spawning two barons of Hell and giving you a soul sphere.
In the building in the northwest in the first room there is a door in the southern wall with a blur artifact.