MAP11: Sewer Shutdown is the eleventh level of Memento Mori II. It was designed by Eric Sargent and uses the music track "Tempter" by Mark Klem.
According to both the text file and the Infopack:
You need to get into the city and check for signs of life. The best route is through the sewers. By the time you reach this point, you should be at the heart of the sewer processing facility - and the city. Since the demons have invaded, little is known about the state of this facility, but this image from one of our scouts may help. We do know it is not 100% operational. It must have been damaged in some way. You must reach the central point of the processing facility and shut it down. Once the system has been shut down, you should be able to obtain access to the city.
Immediately after entering the sewage tunnel from the map start, go north and ascend the stairs to the west. In this room, some metal panels in the north are out of alignment with the rest. Open this wall (sector 196) to encounter four imps and to find a room with a box of shotgun shells and a switch that fixes the broken bridge, allowing access to secret #2. Note that if you drop from the broken bridge, fixed or not, you will become unable to access this secret, as well as secrets #2 and #3, unless you die and respawn during a cooperativemultiplayer game.
After fixing the bridge via secret #1, cross it to reach another room (sector 121) with three imps and a soul sphere.
Within secret #2, there is a wall (sector 225) marked with a "Poison" sign. Open this to find a small toxic pool, around of which are a box of bullets, a box of shells and a duo of imps.
After dropping from the bridge and proceeding into the next room to the south, enter the western passage and the room northwest of it; you will eventually come across a switch resting on a curved, silver wall. Flip this switch to open a secret compartment (sector 140) behind you, containing a chaingunner (not on easy skill levels), a box of bullets, a box of shells and three health bonuses.
After the yellow keycard door, press on the northern wall with the skull upon it to find a small compartment (sector 178) with a green armor and a medikit.
In the brown room following the red keycard door, part of the eastern side of the room has faulty lighting. Open the wall (sector 221) here to find a stimpack, two armor bonuses, a box of bullets, a box of shells, a chainsaw and a switch.
Flip the switch in secret #6 and the western wall (sector 214) will open, releasing five (three on easy skills) imps in front of a backpack.
At the north of the final sewer area, take the western teleporter. After this, push the southern of the two skulls on the eastern wall to open it and enter the teleporter. You will be taken to a tiny hallway (sector 38) with a megasphere on the western side, and both a box of rockets and a berserk pack on the other. You can only teleport here once, and entering the eastern side first will void your chances of obtaining the megasphere, as you cannot enter the teleport pad from the eastern side without warping. Instead, enter the western side first, as you will not be teleported if you walk onto the pad from the western side, and then obtain the other two items before warping back.
In the westernmost part of the level, near secret #5, there is a switch (Linedef 639, Type 23 - S1 Floor Lower to Lowest Floor Tag 3) which allows access to the red keycard. It is surrounded by a silver stand (sector 59) which prevents the player from stepping next to the switch. Nevertheless, the stand is too thin in terms of X and Y coordinates and the switch still can be activated from a distance. As it is not necessary to flip a nearby switch (Linedef 656, Type 71 - S1 Floor Lower to 8 above Highest Floor, Tag 2) lowering the silver stand, which is hidden behind the yellow key door, it is possible to finish the level without picking up the yellow keycard.