MAP21: Sanctuary is the twenty-first level of Memento Mori II. It was designed by Andy Badorek and uses the music track "Dredge" by Mark Klem. It contains a separate area for multiplayer competition.
According to both the text file and the Infopack:
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.
After passing the blue door, you eventually see a soul sphere guarded by a chaingunner at the north of the level. You can lower the metal pillar (sector 218) to its north like an elevator. Take it up and pass through the opening to grab the soul sphere.
When you enter the northern walkway in the starting area through either red key door to its north, an alcove with (two pain elementals on medium and hard skills,) four cacodemons and four lost souls will be revealed in the northeastern 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 (sector 277) with a medikit and a bulk cell.
At the top of the stairway north of the western red door mentioned in secret #3, climb onto the northern railing and then jump to the platform in the north. Open the next two doors to find a switch; flip it and the revenant's pillbox (sector 187) (on medium and hard skills there is also an arch-vile inside) to the north will open. (The chaingunner's platforms northwest and southwest of the chaingun are lowered also.) Jump out the window and enter the pillbox to collect a box of rockets and a rocket launcher.
From secret #3 go straight westwards, riding up the platform and passing the red door to the west to enter the room where you have to lower the green pillars blocking the yellow skull key room; you can walk through the northeastern door, upon which an alcove will open in the southeast. Head to this alcove, and look to the south to find that the easternmost section of the southern brick wall is not worn out. Open this to find (a medikit on easy skills,) a clip, a computer map and a switch (sector 333).
Flip the switch in secret #4 and return to the aforementioned green pillar room to find that you can now access the multiplayer portion (sector 386) of the level. (The wall northeast of the elevator platform is also lowered.) After killing two cyberdemons and seven imps you can obtain four medikits, two boxes of shells, two boxes of rockets and a bulk cell.
After getting the yellow skull key, you have to use an elevator. There is a recess to its north. Open it when the elevator is lowered to find a soul sphere (sector 331). Picking it up opens the southernmost part of the wall in the western three imps' inset, releasing an arch-vile who teleports to the area surrounding the yellow skull key. Enter his hideout to be lowered to a room containing four (two on easy) chaingunners, an energy cell and a medikit. Ride up the lift to the east back to the yellow skull key area.
The platform (sector 16) with 4 sets of shells and, more importantly, the blue skull key can be lowered by pressing a satyr switch (Linedef 316, Type 102 - S1 Floor Lower to Highest Floor, Tag 2) reached after completing the first part of the level, but also by pressing 'use' on the eastern part (Linedef 28, same type as the former) of the southwestern metal pillar that is part of the four metal pillars east of the platform.