Level by Opaque79
Walkthrough by Phil Lambeth (see also Doggett D. McDogg's video walk)
You're faced with an immediate wall switch puzzle. Run forward and pull down the 1st, 2nd and 4th switches (looking from left to right) to open the trap door behind you. Drop down the hole and enjoy a long slide down to the sewers. Shoot two rats from the safety of the ledge, hop down and loop around left to find a wall switch that opens another nearby trap door. Hop down into the shallow water, wade past a closed door requiring a key, and simply drop down the waterfall into much deeper water below.
Pull out quickly and deal with a pair of crocodiles. Jump back into the water and ignore for now the W opening. Locate the N floor hole and swim down. Follow past a closed door and surface. Pull out into a room and shoot two rats. Drop down the other hole and find four wall switches. Pull down only the N one and the E one to open the door in the room above. Pull back out and go on through to find three ramps with spikes separating them. Hop the gap, run up the middle ramp until three boulders are triggered, then jump right or left to an adjacent ramp and wait for the boulders to crash to the floor. The door at the top of the middle ramp opens, allowing access to a wall switch that opens an underwater door (not the one you saw earlier).
Go back, jump into the water hole and return to the large basin. Turn right and go through that W opening you ignored earlier. Follow past the opened door and pull out into a new room. Grab the shotgun ammo you can see to your left and shoot the crocodile lurking just beyond the arch. You can see a closed door in the upper NW corner. Get behind the blocks in the lower NW and find a timed wall switch that opens that door. Pull it, hop back while angling to your left, jump forward and run to the N wall, reverse roll and jump S onto the short block, turn right and pull up left onto the higher block, side flip right onto the next higher block and hop forward through the timed doorway. Run up the ramp, turn the corner, slide and jump the gap, and pull down the wall switch to close a door and raise the water level in the sewers.
Turn around and drop down through the gap. Go up to a ledge that crosses the sewer room and leads to an opening in the SE corner. Jump into the water here and search the SE ledge for a small medipack. Surface, pull out and go through the SE opening. You come to a drained pool. Pull back the pushblock two times and use it to get up S to a ledge with a wall switch. Pull it, hop down to the block and jump to the N ledge for shotgun ammo and a large medipack. Drop down, enter the opening vacated by the pushblock, look left for revolver ammo and exit N.
When you reach the water, jump in and swim down into the E opening. Follow around to the large basin where you shot the two crocs and surface. Pull out onto the E bridge and run to the far end for a small medipack. Jump back into the water and swim through the N opening that you couldn't reach earlier. Swim up the steps, surface and pull out. Run forward to the pushblock and push it into the next room. Go around to the other side and push the block under a timed wall switch so you can climb up and pull it down. Hop down and quickly pull the block once again so you can use it to reach the ledge over the wall switch. Pull up, take a rolling back flip to the higher block and run N through the timed doorway.
In the next room find a water hole in the NW corner, jump in and follow the passage, keeping the wall to your left until you reach an underwater ceiling switch to pull. Go back the way you came, again keeping the wall to your left, until you reach an opening to your right with a second ceiling switch. Pull it down and grab some air in the nearby ceiling hole. Go back, still staying to your left, until you reach an opened door leading to a third ceiling switch. To exit, swim straight out and grab some air if necessary, then go S, take the second left at the wall, swim straight to the E wall and turn left, then follow to the exit hole and pull out.
Note the closed NE door and climb the blocks near the SE corner, jump over W and keep climb until you reach a wall switch. Pull it down to open that NE door, which doesn't appear to be timed. Slide backwards down the N slope, grab, pull up and take a back flip to the ledge and pick up the SHOTGUN. Drop to the floor, go the opened NE doorway and find the RUSTY KEY. Exit S, drop down to the lower floor and jump S into the water. Don't swim down but rather pull up E and ramp up the ramp to find a receptacle for your Rusty Key. Draw your pistols or shotgun as the door opens to shoot a trio of advancing rats.
Run along the ledge and turn right at the corner. Continue around and up a ramp and enter a new room where three more rats are waiting. Locate the opening near the NE corner and run up more ramps to an upper level. Run across to the dark W wall and pull up into the high crawl space there for the only SECRET in this level. Crawl forward, pick up the REVOLVER and crawl back out. In the room below shoot two rats (if they hadn't greeted you upon arrival) and enter the N passage to find a host of wall switches. First, pull down the E wall switch, then pull down the NE wall switch and push it back up again. Now push the E wall switch back up and exit this room. A trap door has been lowered in the center of the previous room, so hop in and slide down a slope to a lower passage. Follow to an opening and jump SE to a central ledge. Turn left and jump NE to a balcony.
Pull up E two times and pull down the wall switch. Jump back to the central ledge and find the SEWER KEY. Spikes appear on the balcony behind you, so jump SW to the alcove, pull up and slide down S. Follow down the ramp to the water, turn left and follow the ledge W to the opened doorway and down the next ramp. Jump into the water, swim S down the steps and back to the large flooded basin. Pull out E and use the Sewer Key to open the door. Enter and slide down two long slopes to a room guarded by two Atlantean centaurs. You'll find revolver ammo on the steps that replenishes itself when you run out. Climb onto the central block. When you stoop to pick up the ANKH, the level ends.