From the start, go through the tunnel to the left. In the room at the end of the tunnel, open the tan colored pentagram on the wall to reveal a blue armor.
In the room with the blue-key door, turn around from the door and look to the south. From the pillar you see there, walk strictly to the west. At the end of the hallway, push the northern part of the western wall to reveal a room with a chainsaw and a backpack.
In the hallway where you first see the blue skull key, open the differently textured wall to the left of the exit door to reveal a soul sphere.
After picking up the blue skull key, go to the hallway north of it. Open the differently textured wall to the right to reveal a room with a rocket launcher.