Run past the doors that lower revealing zombies, head for the silver key ahead, it will drop trough the floor, turn left, then right, and jump across the gap for a grenade launcher. Drop a grenade on the zombie below, then jump down for the 100 health bonus in the secret area. Get the lift back up, and return to the zombies, kill them and press the button in their room.
Enter the new hallway to the right, get the nail gun and go to the lift on the other side of the room. Ride it up and kill the scrag at the top. Follow the ledge around and get the yellow armour and the other scrag. Press the button, then drop through the hole that is revealed. Quickly deal with the ogres and then head down the path over the lava, blasting a zombie and two more ogres. An ogre is behind you on a platform, deal with it, look down at the wall in the pit below. One of the walls has a raised profile. Shoot the wall above that, near the lift, at the wall should open, jump in to claim the ring of shadows in the secret area. Go through the portal. Now head up the steps and the room descends, kill an ogre to the left, press the button, and kill another behind you, Enter the passage to your left, battle another ogre and press the button. Go through the portal back to the main room. Two fiends are in the room below, try to kill them from the safety of the ledge. Then jump down and press the button, zombies appear behind you, kill them, then look at the wall opposite the button, there is a discoloured area on the wall. Hit it, then go through the portal to another secret area. Jump across to the other platform to grab a few goodies. Now return to the pit and proceed through the open door. Enter the room beyond, and deal with an ogre, grab the silver key and go through portal. Enter the silver key door, and gold key door. Kill the ogre guarding it, then press the button in the middle of the columns. Kill the zombies and scrag, lob grenades through the holes above the shambler pit to kill it, then step on the floor switch. Get the double-barrelled shotgun. Press the button, get the gold key and go through the portal. Go to the gold key door and walk through the rune gate to exit the level.