The yellow skull key is located on a platform that looks like a teleporter pad. This platform is surrounded by a stone ledge. As soon as you cross one of the four borderlines between the normal ground and the stone ledge, the hall entry door will shut and trap you in the hall. Crossing the line between the stone ledge and the teleporter will lower four columns, releasing monsters (at least on medium and hard skill). To avoid being trapped with the monsters, you can lure one of the imps from the hall to the outside area, then go back into the hall and wait near the stone ledge for the imp to come back. As soon as the imp stays in the door, carefully enter the stone ledge, but do not enter the teleporter pad. The door will try to shut, being blocked by the imp. Kill the imp(s), then again enter the stone ledge crossing exactly the borderline you did before (each one of the four borderlines between the hall ground and the stone ledge only once triggers the door to shut, so crossing one line a second time will do nothing (function: W1 Door Close (fast))). From here you can enter the teleporter, picking up the yellow key, and go back outside to take cover. If you left ammo outside, you can come back later and pick it up. (In coop mode of course, one player can wait outside - while another one triggers all four lines - and simply reopen the door by pushing the western switch.)
At the east end of the map is an outdoor area with a soul sphere atop a high pillar. On the ground, make your way through the opening in the "C"-shaped brown brick wall, next to the nearby grey stone pillar. Climb the "ladder" on the inside of the brick wall and follow the path up to a blood-filled room. At the west end of the room is a shaft which leads back outside. Fall through the shaft and quickly run over to the aforementioned grey stone pillar. A switch has been temporarily revealed; flip it to lower the soul sphere at the far side of this area.