Level 55: Fortress of Mystery

Upon entering, you will be surrounded by 4 Barons of Hell. Defeat them using your preferred weapons; a Plasma Gun with ample ammo is recommended.

After defeating the Barons, 8 openings will appear in the room. Collect the Shotgun, Chaingun, Rocket Launcher, Plasma Gun, Backpack, and Computer Map (which reveals the layout on your minimap (a small map displayed on your HUD (Heads-Up Display))) from these openings.

Head to the next opening, which contains a door.

Open the door and proceed into the next room. Defeat all the Cacodemons within this area.

If you run low on ammo, collect the Box of Rockets, Energy Cell Packs, and Box of Shells available in this room. If you need health, grab the Medikits, Berserk power-up, and Soulsphere.

Once all Cacodemons are dead, collect any remaining health and ammo.

From the location of the door you entered this room from, proceed straight to the opposite end until you reach a wall.

Open the wall and go through it.

To your left, you will find a switch. Activate it to open the adjacent door, revealing the Blue Key.

Grab the Blue Key and use it to open the Blue Door.

Inside, collect the Red Key.

Use the Red Key to open the Red Door.

Inside, collect the Yellow Key.

Use the Yellow Key to open the Yellow Door.

Inside this final room, hit the exit switch to complete the level.