At the very end of the level, there are two exits. One will take the player straight to the next level, the other will send the player to telefrag a barrel on a type 11 Damaging floor. The latter will still send you to the next level, but with very low health. Looking closely, the regular exit (on the left) is textured black metal, and the harmful exit (on the right) is textured a dark bloody red.
After riding up the first lift, go to the eastern hallway, push the switch at its southern end to open the door in the north that you enter then. Ascend the stairs, kill the two hell knights, and push the door to lower the stairs. Kill the revenant to your right. The alcove he was lurking in has a bloody back wall. Open it to get a backpack.
In the T-shaped room with the cyberdemon and the yellow and blue keys, two of the tan "canvases" on the southern wall can be opened up: Push the second one from the east (with a medikit in front of it) to open the room you saw in the north of the first lift you rode up in this map, containing a rocket launcher, two rocket boxes and a medikit.
In the same room where secret #2 is accessed, open the canvas left of the yellow door. The room behind contains two bulk cells and a megasphere.
Press the green torch in the "library". This will open an alcove with a berserk pack in it west of the skull switch that raises the walkway to the blue key.
In the same "library", push the back wall of the southern cage opposite the green torch to lower the wall behind the northern cage. Go trough the opening to register the secret and open the back walls of both cages. To your right is a medikit and a blue armor. Continue along the ledge to the left until another room opens up, containing two lost souls, a rocket box and a energy cell.
After riding up the first lift, go west and climb over a well's edge to fall down on a soul sphere. Pick up the four enegy cells and climb the narrow stairs at the western wall, push the skull switch to lower the plasma gun and the southern wall that releases some revenants. After caring about them, collect two more energy cells from their hideout. Ascend the stairs to find a chaingunner and a bullet box. Push the switch to open an alcove with a blue armor, the exit and alcove on the northern side of the hallway that contains a medikit and a revenant. Returning into the hallway also opens a secret room to your right with some chaingunners, a super shotgun, a box of shells and three medikits.
During development this map was simply called "BloodWall".
This map offers a choice of two portals in the exit room. The left portal will lead directly to MAP29: Ticket to Eternity, while the right portal will teleport the player into a completely enclosed room with a damaging floor of Type 11, telefragging a barrel as they go, in which the player will not exit the level until health drops to 10% or below. If the right portal is chosen, the player will begin MAP29 with extremely low health. This trap is similar to the one in MAP11: Hunted of The Plutonia Experiment (minus the arch-viles).