This level occupies the map slot MAP21. For other maps which occupy this slot, see Category:MAP21.
As you leave the cage you start the map in, push the wall under the skull switch to your left to lower the pillar (sector 51) with the megasphere on it in front of you behind the fence. Drop down into the southwestern blood pool later to grab it. You have to step on the pillar's center to register the secret.
From the map's central circle take the western route. Behind the bridge with the 16 armor bonuses, push the wall under the southern skull switch on the western wall to open a niche (sector 10) between the skull switches with a BFG9000 in it.
Opening the door (sector 108) on the eastern route to release the monster behind it (thing 140: mancubus on easy skill levels or thing 137: cyberdemon on medium and hard skills), not killing it and continuing the map may result in keeping some enemies trapped forever: Using the switches with the linedeftags 3, 4 and 5 let several enemies teleport to several locations. If the sector 15, 16 or 17 is occupied by the aforementioned monster at the moment when the enemy tries to teleport it, it will never be teleported since the corresponding linedef action is 125 W1 Teleport (Monsters only). The teleportation only works if the enemy crosses the linedef for the very first time and if no monster is on the target sector.
Because of the high probability that some of the arch-viles will resurrect some monsters, the kill count may exceed 100% anyway on most source ports, but after killing any enemy on the map and standing in front of the exit, one may hear several monsters grunting. Using the iddtcheat shows the remaining monsters moving around in their starting places, unable to teleport in.
To avoid this, try to kill any monster as soon as possible before using any switch.