E4M1: Butchery Loves Company
Drop down (note the Skull key door to your right), go up the stairs behind you, and through the door. Run up the steps, grab the Skull key, retrace your steps, and go through the Skull key door. Go up the steps, and follow the hall to the second passage on your right. Go up the steps, across the wooden bridge, open the door under the "Brain Storage" sign, and grab the Eye key.

Make your way back to the hall, turn right, and follow the hall around the corner to the Eye key door. Walk through the electrical beams to your left, and follow the hallway to the room at the end. Across from the third cage is a metal door. Go through it, and push the symbol to end the level.
E4M2: Breeding Grounds
Go up the steps, and walk all the way around the aquarium. You'll come to a room with the Skull key door to your right, and a metal double door in front of you. Go through the double doors, and kill the innocent in the next room to collect the Skull key. Return to the Skull key door, and go through it. In the next room is an Eye key door, and a spiral staircase. Take the stairs up, through the metal doors. After going through the metal doors, take the staircase to your left.

When you get to the top, kill the cultist to get the Eye key. Go back downstairs to the Eye key door, and go through it. Follow the corridor, up the stairs, and you'll find yourself facing a pool and another room to your right. Dive into the pool in front of you. Follow the underwater passage to your left, shoot open the vent, and keep swimming down the passage. You'll eventually find the symbol to end the level.
E4M3: Charnel House
Swim forward, and surface. Jump out of the water tower, and enter into the building through the metal door. You'll find a locked door, and window overlooking a bloody pit. Jump into the pit, and turn the switch. Now open the metal door, and go into the furnace area. Go through the passage in front of you, and take the second passage to your right, and you'll find yourself in a passage with a stream of water. Follow it down to the room with the staircase. Go up the stairs, and at the top of the stairs, follow the passage to your right. You'll find yourself in room with three arches in front of you and a hole over to your left. Go through one of the arches, and you'll find an aquaduct.

Jump in and follow the flow. After you drop down, take the underwater passage forward into a larger area. Turn around, and you'll see a TNT barrel next to a pillar. Blow it up, surface, and jump on top of the pillar into aquaduct. At the end of the flow is a switch. Turning it opens up several doors, the first to your immediate right. Jump down and take the newly opened underwater passage. Follow it, surface, and jump into the area before you. To your left will be a waterfall, and to your right is a ramp leading up to the furnace. Take the ramp up, and you'll find yourself in front of fire. Quickly open the door to the right of the flame, and you'll find yourself in the furnace room again. Turn left, and walk all the way down to the wall. Make another left, and walk all the way down to end. Take the ramp to left up, then take the passage to your right.

At the first fork, go right. Follow it all the way down, and turn to your left. Go outside and you'll find yourself on a ledge. Turn right, and follow the path up the stairs into the water tower. After entering the water tower, you'll see another exit to your right down a wooden aquaduct. Jump on to it, and be careful not to fall into the hole at the end of it. Turn to left and take the passage all the way down to the symbol that ends the level.
E4M4: Crystal Lake
Walk forward, and enter the barn to your left. After entering the barn, go to the left, and pull the lever to ride the lift up to the 2nd floor. Head toward the right, jump down, and follow the path over to your right. Go through the metal door, up the steps, and through another metal door into the dining room. On the far side of the dining room, you'll find another passage leading to a room with a fireplace. Go through the passage across from the fireplace, and out the door to your right. Turn to your right, and follow the path around the house, into the woods, across a bridge, and you'll eventually find another building.

Jump up on the porch, and enter the door on your left. Grab the Moon key, and go back outside. Continue along the path to your left, and you'll get to the outhouse requiring the Moon key. Open it up and jump in. Sink all the way to the bottom, and follow the underwater passage. Follow it to the concrete underwater waterpassage, and follow it to your left. After turning the corner, take the second passage on your left. Swim to the end of it, turn the switch, swim through and surface at end. Jump up on to solid ground, and you'll see the exit symbol in front of you. Don't hit it. Instead, turn to your right, and shoot the wall a little bit to the left of the water. It's not marked, so look around. After blowing the hole in the wall, enter it, and push the symbol to go to the secret level.
E4M9: Mall of the Dead
Walk forward into the next room, and go up the escalator in front of you. Turn left, walk to the wall, turn left, and go up the ramp. Enter the K-Marche, and go into room to your right. Kill the stone gargoyle to get the Skull key. Go back outside the K-Marche. Turn to your right, and follow the passage around to the Skull key door. Go up the steps, and turn right.

You'll see a couple of vents. The one in front of you a secret area (use the boots to get to it), so jump up into the one to your right. Follow it to the store room. Over to the left of the store room is a stack of crates leading up to a hole in the roof. Jump up into the roof, and push the symbol to end the level.
E4M5: Fire and Brimstone
Walk forward, and jump across the lava onto the path. Follow it along to the end. Turn to your left, and jump across to the part of the path. As you get to the end of the cave, you'll find a ramp leading up to your right, so follow it. It goes on for a while, but you'll eventually cross a short wooden bridge. To your left, is a pillar with a voodoo doll on it. Jump on it, then across to the next ledge, and then again to the next ledge. Turn to your left and follow the passage.

You'll eventually come to a room with the Skull key. Just as you enter the room, you'll see a staircase on the left side of the room. Grab the key, and go up the stairs. Follow the path along, across the narrow ledge into the building. Walk forward, and go through the Skull key door. At the end of the room, take the wooden bridge to your left, and follow the ledge into the temple. As you come into the pillared room, you'll see a stone design behind the altar. Open it, and push the symbol behind it to end the level.
E4M6: The Ganglion Depths
Follow the passage in front of you until you get to the iron gate. Push the skull symbol next to it to open it. Continue along until the large wooden door, and use the skull symbol to open it. Walk to the iron gate, and you'll see the Fire key door to your right. Enter the passage to your left, take note of the Skull key door to your right, and continue to the end of the passage. Open the ornate door, and you'll see a metal pillar in the middle of the room, and two skull symbols past it. Push both of the skull symbols, and the pillar will lower revealing the Skull key. Grab the Skull key, and quickly run out of the room to avoid the fireballs. Enter the Skull key door.

Go down the steps, and you'll see the Eye key door to your right. Take note of it, and go through the wooden doors on the other side of the room. Go down the stairs, pick up the Eye key, and return to the Eye key door. Follow the passage around to a wooden door, and go through it (there's a fire behind the door). Just keep following the passage along, up some stairs, and you come to a room where a door will open and a gargoyle will attack you. Follow that one, down some stairs, and you'll eventually come to a room with the Spider key door. The Skull key is in the room next to this one. Go through the Spider key door, and grab the Fire key at the end of the passage. Push the skull symbol next to it, and step onto the wall that lowers down. Ride it back up, and you'll find yourself next to the Fire key door. Follow the passage until you get outside.

 Follow the stairs down, and you'll come to a narrow ledge. Follow the ledge along, and jump across the gap. Follow the passage along, up the stairs and such, and you'll come to a lift. Take the lift up, and walk around to another lift. Take the lift down, walk across to another lift, and take that one down. Follow the passage around until you get to the bridge that blows up. Jump across, and the next bridge blows up. Jump across that one, and follow this passage along until you get to the symbol that ends the level.
E4M7: In the Flesh
Go down the fart elevator to your left. Follow the passage to your left into the next chamber, and go half-way down the passage to your left. Turn to your right, and follow the ramp down. As you go down, the pillars will lift up (not important to the walkthrough, just mentioned as a landmark). Jump into the green slime, and swim forward through the small hole in the wall. Don't surface in the next room (you'll come back to this room later), but continue on into the hole you see in front of you. Surface, and walk into the narrow path with the gillbeats in it.

 Push the skull symbol at the end of the path. Now jump back into the green slime, back through the hole you came through, and now surface in this room. Behind you, you'll see a passage. Follow it, and push the skull symbol. Turn around, walk back down the passage, around the two pools, into another passage. This leads into the heart, and as you make you way around the chambers, you'll find a hole in the wall. Jump into it, and push the symbol at the end of the passage to end the level.
E4M8: The Hall of the Epiphany
Enter into the temple. You'll see a skull in front of you, and a room to your left and to your right. Shoot the skull, and enter into the room to your left. Kill the stone gargoyle, then go to the room on the other side. Kill the giant spider, and a couple of more walls will lower revealing Cerberus. Kill him and enter into the throne room with the stone pillar. Push the pillar, and face down Tchernobog. Kill him to end the level and the game.