Level 12: Altar of Pain

From the start, go east through the opening and take the teleporter. Head up the stairs to the left. At the top, go to the northeast part of the square walkway and hit the switch on the pedestal.

Go to the southeast part of the platform, head down the stairs, turn right/left, and you'll see a Red Door. Turn around from the Red Door to face a wall. Walk up to it.

Go through the opening to your left and follow the hall to a Blue Key. Pick it up, go back through the opening, and follow the hall to another opening.

Return to the square walkway and head up the stairs to your left. At the top, go to the northwest part of the walkway, killing any Imps. Open the Blue Door and go through.

Follow the right-hand wall until you hear a door open. Turn around, follow the right-hand wall to an opening, and go through.

Enter the small opening, get the Armor, and hit the switch. Turn around and leave the opening.

Back in the main area, turn right and follow the left-hand wall to a Plasma Gun, killing any Hell Knights. Pick up the Plasma Gun; a switch will appear. Hit it.

Return to the Blue Door, open it, and go through. Hit the switch in front of you; the lift to your left will lower. Ride it to the top.

At the top, turn right and follow the left-hand wall to a switch. Hit it.

Turn left and follow the right-hand wall to another switch, killing any Imps or Barons of Hell. Hit the switch and drop to the floor below.

Return to the Blue Door, open it, and go through. Follow the left-hand wall to an opening and go through.

Enter the opening across from you. Go straight to a pedestal with a switch. Hit it.

Go left into the opening in front of you. Kill the Imp and hit the switch to your right. The lift to your left will lower. Ride it to the top.

At the top, kill the Imps and get the Red Key. Hit the switch to lower the lift again and go back down.

Go through the opening to your right. Go straight to a pedestal with a switch. Hit it. Go all the way left to another pedestal with a switch. Hit it and turn around.

Go through the opening in front of you, then through the opening to your right, and head down the stairs.

Go through the opening in front of you. Walk up to the wall, turn left, and follow the hall to the Red Door. Open it and go through.

Get the Laser in this room and kill all the Barons of Hell. A switch will rise where the Laser was. Hit it and leave the room.

Back in the main area, follow the right-hand wall to a switch. Hit it, and the lift to your left will lower. Ride it to the top.

At the top, go left until you reach the Yellow Key. Pick it up.

Locate the platform with the Soulsphere. Back up and run to get onto the platform. Pick up the Soulsphere.

Switch to your Automap. The platform is arrow-shaped, pointing to a wall. Face that wall. Back up and run towards the wall it's pointing to; you will go through it.

In this secret area, kill the Baron of Hell. A teleporter will appear on its body. Step on it to exit the level via the secret exit.