Okay marines, you are in for trouble. Massive enemy activity has been reported in your destination area, strategically important places have been occupied by large numbers of enemy forces.
Our scout team has reported that enemy forces concentrate around a place called SANCTUARY, which seems to be of special interest to the enemy. Unfortunately the scout team could not get close enough because the entryway was locked and surrounding buildings were well guarded.
Your task is to jump in and clear the area of enemy presence, then proceed in order to find SANCTUARY, and, if possible, find out why it is important to the enemy.
The scout team has discovered abandonned [sic] ammonition [sic] stocks in several places but you are advised to keep count on your ammo or you'll end up throwing stones at those beasts. You may expect massive opposition and ambush attacks.
Before we lost contact with our scouts they reported a sudden rocket attack in a large square room in the most western part of your destination area. It is probably better to avoid that room if you want to make it home in one piece.
At the north of the level, you can press upon the metal pillar to lower it like an elevator. (sector 218) Take it up and pass through the opening to get a soul sphere.
When you reenter the starting area through the red key door south of the southern stairway, an alcove will be revealed in the northeast corner. Drop into the water and use the stone elevator; take it up, and jump into the alcove. Open the metal section in the north to reveal a secret tunnel with a medikit and a bulk cell. (sector 277)
At the southern stairway, climb onto its edges from the top, and then jump to the platform in the north. Open the next two doors to find a switch; flip it, and the Arch-Vile's pillbox to the north will open. Jump out the window, and enter the pillbox to collect a box of rockets and a rocket launcher. (sector 187)
In the room where you lower the green pillars blocking the yellow skull key room, you can walk through the eastern door, upon which an alcove will open in the southeast. Head to this alcove, and look to the south to find that a section of the brick wall is not worn out. Open this to find a clip, a computer map, and a switch. (sector 333)
Flip the switch in secret #4, and return to the hallway where the green pillars are or were to find that you can now access the multiplayer portion of the level. (sector 331) Here, you can obtain four medikits, two boxes of shells, two boxes of rockets, and a bulk cell.
In the yellow skull key room's elevator, there is a recess in the north. Open it when the elevator is lowered to find a soul sphere. (sector 386)
The platform with 4 sets of shells and, more importantly, the blue skull key can be lowered by pressing a satyr switch available after completing the first part of the level (linedef 316), but also by pressing 'use' on the eastern part of one of the metal pillars supporting the ceiling (linedef 28).