Level 20: Unholy Cathedral

Open either door in front of you and proceed through.

Turn left and follow the hallway to a wall with a star symbol, which is a door. Open it and go through.

Step on the teleporter to be transported to the center of the courtyard.

A door next to you will open, revealing a Baron of Hell. Kill it and collect the Box of Shells and Invincibility Sphere from the secret room.

Return to the center of the courtyard. In each corner are 4-pad teleporter sets. The teleporter in the northwest corner will advance you.

Use the Automap (a map displayed on your Heads-Up Display or HUD (heads-up display — the on-screen icons showing health, ammo and the minimap)) if you cannot find it, then step into the northwest teleporter.

Kill all the Demons in this area.

Grab the Medikits if needed. At the far end of this room are two doors; one to the right and one to the left.

Open the door to the right and go through it.

Turn right and follow the hallway until you see a silver wall on your right.

Next to the silver wall is a pillar with an opening. Walk through the pillar (facing the door) and the wall will lower, revealing an opening. Go through it.

In this room, there is a switch in each corner. Hit the switch in the top right corner.

Go through the opening to your left.

A platform with the Blue Key will lower. Grab the Blue Key, turn around, head right, and leave this room.

Back in the courtyard area, kill the Baron of Hell to your right.

With your back against the opening you just came out of, turn right and follow the hall to a wall with a star symbol (a door). Open it.

Step on the 4-pad teleporter set. It will bring you back to the room where you previously fought Demons.

Open the door to the left this time.

Kill all the Imps and follow the hallway until you see a Blue Door to your left. Open it and go through.

Open the exit door to your left and go through it.

Step on the exit teleporter.