After the two spider masterminds are dead, run onto the (northern) switch platform, and open the eastern wall with bloodstains on it to find a berserk pack (sector 131).
After the two spider masterminds are dead, run onto the (southern) red skull key platform and open the western wall with bloodstains on it to find an armor bonus. Take the elevator down, and then lower the next elevator (sector 324) to acquire a green armor.
Walk onto the bridge to the yellow skull key from the south. A chunk of the wall will raise on the walkway near the door. When walking across the space it was on, the floor under your feet will be lowered to a compartment containing a group of imps, two sets of shells and a stimpack. Kill the imps, push the switch to call the elevator and ride it up again. Hugging the southern wall, walk to the east to lower the elevator again and to reveal a switch to your right. Flip the newly-revealed switch, and drop down the elevator. Enter the additional elevator that has been lowered, ride it, and open the metal door to enter a cage (sector 260) containing a soul sphere.
In the room with the four teleporters in the north of the map, shoot the monitors to open a compartment containing three sets of shells, a stimpack and an arachnotron. Kill the arachnotron and then open the wall with the triangle upon it to find a hidden corridor (sector 461) followed by a section of nukage, containing a megasphere. Go further down the corridor to reenter the sewage area.
There are two sets of shells (things 160 and 161) in the compartment where players #2 and #4 spawn. The bars (sectors 4 and 7) blocking the entrance to the compartment can only be opened (Linedefs 20 and 21, Type 112 - S1 Door Open Stay (fast); Tag 74) from inside. Thus, the shells are not collectible in single-player mode, but are not flagged as multiplayer only.
Areas / screenshots
Routes and tricks
If you edge very slowly toward it, you can grab the yellow skull key without triggering the chaingunner trap behind you.