E3M1: Ghost Town
In front of you is the Fire key door, take note for future reference. Walk forward, and turn to your left. You'll see a gray door on the second building on your left. Enter it, and you'll see the Spider key door. Turn right, and follow the hall around. Kill the cultist to get the Spider key. Go back, and open the Spider key door. Go up the stairs until you reach the stucked door. Shoot the fire extinguisher, and a hole opens up into the next building. Jump over to the other building, go up the stairs, past the innocents, up another flight of stairs, and you'll find another hole in the wall. Jump into the hole across the alley. Go up the stairs, through the gray door, and ignore the "Stay Out" sign. Enter that room, and kill the ghost to collect the Moon key.

 Go back out to the street, and you'll see a hole blasted in the side of building to the left of the JoJo sign. Enter it, and another hole blows open. Jump up into it, and you'll find the Moon key door. Enter it, and pick up the Fire key door, and go back to the street. Go through the Fire key door (one of them won't open), follow the hall to your right, and you'll find yourself in a fairly large room. Jump up over the counter, and enter the Manager's office to pick up the Skull key. As you leave the Manager's office, you'll see another door in front of you. Go through it, up the stairs, and through another door. When you pass by the window, look out of it. You'll see a crack in the wall. Blow it up, but don't jump over yet. Continue along the hall , and you'll eventually come to the Skull key door. Go in, grab the Dagger key, and return to the hole you just blew up. Jump across, go through the Dagger key door, and push the symbol to end the level.
E3M2: The Siege
As you enter the second room, you'll see a crack in the wall; blow it open. Enter the hole, and go down the stairs to "Feinmen Meats." Go in through the regular entrance, and make your way forward. You'll find a passage under a "No Admittance" sign. Go down that passage, and open the door at the end to pick up the Skull key. Go back outside, and enter through the Loading entrance (there's a hall that's quicker, but this is easier to explain). Go up the stairs, and follow the hall until you get to the Skull key door. Enter it, and follow the hallway until you get to the bloody pool. To your right, you'll see a white square (take note of this for later). Jump down to your left, and you'll find a button is a small alcove. Push it, and the white square I pointed out will now have a button. Go back to where you jump off, ride the lift back up, and push the newly revealed button. A staircase will rise up, follow it to another button, and push it.

To find out "what the hell" that did, go back to the Skull key door, and follow the ledge around. There will be a newly opened door on the other side with another button. Push it, and the other door on the ledge will open. Enter it, go up the stairs, and out on the narrow ledge. To your left will be a water tower, so go over to it. When you get to it, you'll see a window over to your left. Jump in the window, kill the cultist, and grab the Moon key. Jump back out the window, and enter the door under the "Exit" sign. Walk down the hall and shoot the TNT barrels to blow the crack in the wall. Now follow out to the roof behind the "Monolith" billboard. To the right of the skylight, you'll see a wooden fence you can get on top of. Get as high as you can, than jump across to wall near the black and white tiled area. Now jump to the roof over to your left. To your left will be a fan. Shoot it out, and drop down into the kitchen. Take the right path, and follow it along with your right side against the wall (ignore the small area with the phones), and you'll soon find yourself outside with some innocents. Enter into the building in front of you.

 Follow the path to your right, and after you turn the corner, you'll see the Moon key door in front of you. Enter the Moon key door, and you'll soon see an area will lasers blocking your path. If you look behind you, you see two switches. Activate the right one (don't touch the left one). The lasers will now be deactivated. Enter the room, and you'll see something across from you with a black rectangle on it. Lob some TNT at it, and the bank vault to your right will open up. Inside the bank vault, you'll see two dials. There doesn't seem to be any combination. Just hit one of the dials a couple of times, and all of the metal doors inside the next room open up. Walk into the room, and you'll find a hole blown in the wall. Jump through, and push the symbol to end the level.
E3M3: Raw Sewage
Walk forward, down the stairs, and into the water. You'll find an underwater passage at the end of this corridor. Enter it, and you'll come to a room with two valves. Turn them both, and a passage will open over to your right. Follow the newly opened passage, surface, and go up the steps into another passage. Follow this one until you get back into the sewage. At the first fork, take the left path, and follow it along until you see a hole overlooking a large pool of sewage. Jump into it, and underneath the waterfall (sewagefall), you'll find an underwater passage. Follow it to a larger area with the Skull key and a passage to the right of the key.

Grab the Skull key, and exit through this passage. Follow it, and when you fall down, push the switch next to the gate to open it. Go through the gate, surface in front of the Skull key door. Enter the Skull key door, and follow the passage until you get to a large room with several metal bridges over it. Enter into the lift over to your right, and take it up to the next level. Cross the bridge, and follow the path along until you get to a metal door. Past that door is a room with some bedding and a lift. Take the lift up, and you see the Eye key door over to your left, and a bridge. Cross the bridge and open the door. On the other side of this room is a button which opens causes the circular thing in the middle to spin around, revealing the Eye key.

Grab it, and use it to open the Eye key door. Follow the passage, and you'll see the Fire key and another Eye key door across the bridge. Grab the key, and go through the Eye key door. Follow this passage all the way around, across another bridge, and open up the Fire key door. Push the symbol to end the level.
E3M4: The Sick Ward
Enter the building through the Emergency Room entrance. In the emergency room, there's a large metal door. Go through it and walk to your left. Find the information desk, enter the small room behind it, and kill the fat zombie to get the Eye key. Now just walk up the ramp and through the Eye key door. After going down the ramp, you'll see two passages, one to your right and left. Take the right path to the "Assisted Suicide Ward," and grab the Fire key. Now retrace your steps, and take the left passage. At the end of this passage, you'll find the Fire key door. Go through it, and find the chapel. Here, you'll find the Skull key, and the entrance to the secret level. If you simply must go through the regular exit, use the lift on the other side of radiology.

 When you get off the lift, you'll see the Dagger key door in front of you, and the Skull key door around the corner. Enter the Skull key door, and open the metal door across the door marked "exit." Here, you'll find the Dagger key. So return to the Dagger key door, open it, and follow the passage to a metal door. Open that one, and you'll find the symbol to end the level. To get to the secret level, grab the Skull key (opening the curtains), turn around, and look up. Shoot the little skull, and a passage will open behind where the curtains were. Follow it, and push the symbol to go to the secret level.
E3M8: Catacombs
Make your way forward to the double wooden doors. Go through them into an ornate room, and take the passage over to your right. Follow it down, until you get to the Eye key door. The passage to your right is blocked by bars, so take the passage to your left. Follow this path to a wide wooden door, which opens up to the entrance to the catacombs. Once in the catacombs, take the first right. You'll see two different Skull key doors down this path. After the second Skull key door, the path will go around a corner, and you'll see some candles lit up.

In the second niche with candles, you'll see the Skull key on top of a tomb. Grab it and walk back to the wooden Skull key door. Enter it, and follow it around until you get to a switch. Pushing the switch will open bring you back to the ornate room. Go back toward the Eye key door, and you'll see that the bars on the right passage is no longer there. Follow the passage until you get to the room where the door closes behind you, and you're ambushed. Kill everybody, and go down the passage to your right (it should open up for you, but if not, there's a button in one of the alcoves which should open it up for you).

Go down the steps, and you'll be back in the ornate room again. Go across the room, and you'll see a switch on the right side of the room. Pushing it reveals a spiral staircase to your left. Climb it, and follow the passage along (you can take either fork) to the Eye key. Grab it, and the entrance will close up, trapping you in their with some zombies. After a little while, it will open back up, and make you way back to the Eye key door. Go through the Eye key door, and you'll find an alcove with a lever. Pull it, and a button will be revealed behind you. Push it, and a door will open with the symbol you need to push to end the level.
E3M5: Spare Parts
In front of you is a switch, but you can't activate it yet. Instead go through the door over to your right, follow the path around, and go up the stairs. At the top of the stairs, there will be a locked door to your left and a conveyor belt in front of you. Get on the conveyor belt, and carefully go through the crusher. At the end of the conveyor belt, turn to your left, and go out onto the ledge. Push the switch, and now you can activate the switch from the start. Retrace your steps, push the switch, then go back up to the locked door. After you go through the door, you'll find another locked door. Push the button next to it, and duck. The lights will dim for a little while, flamethrowers will try to burn you, and finally zombies will try to kill you. After all this, the lights will come back on, and the door can now be opened. In front of you will be another metal door with a panel next to it. Don't mess with the panel, and just open up the door. Enter it, and you'll see another panel. Activate this one, and the room will spin.

When the next door opens walk out, and look to your right. You'll see a raised platform with two buttons. Look down, and you'll see a slope. Jump down on it, walk around the back, and ride the lift up. Hit both buttons, and jump back down to ledge, and walk up the ramps to the tubes in front of you. You'll see a lever next to it, so push it. Now turn to your left, and you'll see a door marked "No Admittance." Quickly step into and out of the room, and you'll just miss getting smashed. Now go back toward the lever, and continue following the path into the area bathed in red light. Go through the metal door, and down the ramp. You'll see a door with a panel next to it. Push the panel, and the ramp will fall down revealing a small room with a cultist and another panel. Push that panel, and the door will open, revealing a black and yellow striped door and another panel. Push the panel, open the door, and go down into the next room. On the other side of the room is a switch, which will open up the furnace. After pushing the switch, jump up to where the Guns Akimbo are, and jump down into the hole. Quickly move away from the furnace, and up the conveyor belt (be careful of the crusher). Ride the conveyor belt down, and turn your left when you get into the next room. You'll see another conveyor belt and a lift.

Ride that conveyor belt up to the lift, then take the lift up. Walk across the catwalk into the small office with three switches. Push the left and the right ones (don't touch the one in the middle), and the Skull key will eventually appear behind you. Grab it. As you walk out of the office, you'll see a couple of cultists shooting at you from a newly opened window. Jump in the window, and go through the Skull key door. Go down the ramp, and through a door. Turn to your left, and you see Skull key door. Go through it, and go up the lift. Cross the catwalk, and you'll see, yet another Skull key door. Go through this one, and push the symbol to end the level.
E3M6: Monster Bait
Walk forward, and you'll come to a bridge going over to the ledge across from you. Stepping onto the ledge causes it to break, so quickly jump over to the ledge. Some of the walls will fall away, so follow the path along, across a metal bridge, down a lift, through a large metal, and eventually into a room with several panals. The two panels to your left and right are light switches, and the one on the counter in front of you opens the door to your left. Push it, and enter the newly opened door. At the end of the hall, jump up to the next switch, and turn it. A space opens up next to you, jump down and hit the next switch. Turn around, and you'll find yourself back in the room you just entered a little while ago, and now the other door is open.

 Enter into the newly opened passageway, and turn the switch at the end of the hall. Another space opens up in front of you with another switch. Turn that one, and the space to your left drops down. Enter into the new area, where you'll see some pipes and two switches. Hit both of the switches, and the doors in front of you will slowly open. Enter into the darkened room, and you'll find another switch and large wooden lever. Pull the wooden lever to start up the turbine, and then push the button to lower the lift. Take the lift up, and follow the passage along through the metal door. After going through the door, you'll see a lift to your left. Take note of it for later. Instead, turn to your right, follow the passage, go up the stairs, and you'll find yourself in the turbine room. Look up and you'll see the Moon key out of reach. In between the turbines, you'll find a pair of control panels (one in each location). Activate both of them, and find the red door. Wait a moment, and the it'll open revealing a lift. Take it up, and cross the catwalks to get to the Moon key. Return to the lift I pointed out before. Go up the lift, and follow the ledge until you see the bridge to your right. Cross over it, and through the Moon key door. Go up the stairs, and you'll see a switch (refered to from now on as main switch) in front of you. Now for one of the toughest puzzles in the game. Take note of the three red lines. -Hit the main switch to reveal the dial on the right side -Turn the right side dial twice, and you'll see that the middle red line is now blue -Hit the main switch twice to reveal the dial on the left side -Turn the left side dial once, and you'll see the first line is now blue -Hit the main switch twice to reveal the dial on the right side -Turn the right side dial once, and you'll see the third line is now blue -Hit the main switch, and the door in front of you opens Enter into the room, and you'll see a metal door to your right. Enter into, and follow the passage all the way to the end. To the the right of the box of shotgun shells, you'll find a panel. Push it, and the thing in the middle of the water will rotate around. Retrace you steps and go out the metal door. You'll find that you can now jump on the thing in the middle of the water. Jump on it, and find the switch on the other side. Turn it, and it will rotate again, allowing you to jump to the other side.

Go up the ramp, and through the door. Turn to left, and walk around to the lift around the corner. Take the lift up, and go to your left when you get to the top. Follow the passage through a door into a small warehouse. Make your way to the other side, through a door into a control room. Go through the door on the other side of the control room, and follow it to the lift. Take the lift up to top of the dam. Grab the Fire key, and jump into the shallow side of the dam (to your right as you enter). You'll see some barrels underwater, so shoot them. A hole will blow up in the dam, and the water level will rise. Surface, and you'll see a cave that you'll now be able to enter. Jump into it, and activate Fire key pillar at the end. The door in front of you will open, and jump into the flames to end the level.
E3M7: The Halls of Cerberus
Follow the passage until you get to the metal doors, and go through them. To get Cerberus to come out, you'll need to activate 4 skull symbols. Turn to your left and follow the passage. When you come to the first skull symbol, activate it and climb the stairs to your left. Follow the passage to the 2nd one, activate it, and head down the passage to your right. Follow it over the bridge, and past the elevator shaft, and push the 3rd one. Drop down the elevator shaft. Come out of the shaft, head to your right, and activate the 4th and final skull. Now Cerberus will come out to play in the are to your left.