This level occupies the map slot MAP30. For other maps which occupy this slot, see Category:MAP30.
Exit the building where you start the map in and do not walk over the red glowing floor's edge as this would instantly kill you. Outside, go to the north of the building and register the only official secret to grab a chainsaw and a berserk pack. Return to the building's south and follow the path to the south. Walk towards the evil eye to be teleported to another area.
As soon as you move, you grab the megasphere you have been teleported in. Leave the small red spot you are on to slowly lower the black pillar in the south - this takes more than a minute. Dodge or kill the cacodemons that start attacking you, using the ammo provided by the weapon stacks lying around you: There are each two stacks of shotguns, super shotguns, chainguns and rocket launchers. Each stack contains five weapons, so there is plenty of ammo. The dark damaging floor around the circle where the weapon stacks lay on drains your health and armor, so try to keep on the red glowing letters. As the black pillar with the evil eye on its top has lowered, step on it to be teleported to the boss area.
You end up in a stockpile of items: there are seven shotguns, one super shotgun, 18 chainguns, 50 rocket launchers, ten plasma guns, five BFG9000s, a backpack and a soul sphere. Run to the north and use the two eye switches (nearby a soul sphere and a blue armor) east and west of the stairs to enable the access to a skull switch downstairs. Use the skull switch and straferun upstairs, enter either entrance to the "C"-shaped canyon around the weapon stockpile platform. In the south of the canyon (where a blue armor and a soul sphere can be picked up), quickly enter the northern lift and ride it up. From the lift, walk slightly to the north to rise the weapon stockpile platform. Fire some rockets at the boss's central eye to end the game.
Other points of interest
In the first area, go to the candle westsouthwest of the evil eye and look at it. Note that the edge of the red star surrounding the evil eye has an animated texture, except the part of the edge you are looking at. Approach the evil eye by walking over this part of the edge (Linedef 454, Action: 121 - W1 Lift Lower Wait Raise (fast), Tag 5) to temporarily lower the black pillar in the south of the area you are about to teleport in. Lowering the pillar this way only works once and takes only about eight seconds, but the pillar is about to rise again quickly, so enter it fast.
Go outside the walls to the north of the map start building to find a chainsaw and a berserk pack (sector 113).