Warrens is the secret level, but it first appears identical to Hell Keep. The only noticeable difference is a cosmetic change about the size of the gate.
 Replaying Hell Keep
As with the first time you were here, follow the same tactic. Press the switch to head to the ground level, and encounter a few imps. Press the switch on the left to open the door, and draw the cacodemon out into the courtyard, and try to get them fighting each other. Proceed into the corridor; take a left to grab the shotgun (running across the platforms), or a right to proceed to the end of the run.
The next room contains a few imps. The corridor contains a few demons, and it leads to a small room with health and armor bonuses. The room before the exit has imps, the secret rocket launcher, and the final room appears to be the exit teleporter.
Stepping on what appears to be the exit will lower the walls, revealing several boxes of rockets, supercharge, invisibility, invulnerability, and a cyberdemon. You will need to grab the blue skull key, but are free to either engage or avoid the cyberdemon.
Now that the surprise was triggered, the map changes slightly. In the gray imp room, the east wall opens to reveal two cacodemons.
The next room opened to reveal a baron of hell. Past the nukage is a set of lost souls, and a berserk pack, a chaingun, and a radiation suit. To proceed back to the corridor, you will need the blue skull key.
The corridor of demons has a side opening that has specters approach from the sides of the raised platform. Press the switch on the platform to lower it, and access the health, armor, invulnerability and chainsaw. Flipping the switch also releases more specters.
The next room has cacodemons and a plasma gun.
The room leading to the shotgun now has a BFG revealed - jump in the lava, and run around the left hand side.
The starting location has a new pathway opened.
 Back to Start
When you return to the start, head into the pathway, and you will be teleported behind a few cacodemons. When you attack, you will alert a large number of zombie sargents, which can be eliminated by the BFG collected earlier.
Grab the key. As you do, a baron of hell will approach. If you enter the alcove that contained this enemy, the alcove will open further with three Barons of Hell, and a supercharge.
With the red key, you can head to the exit. It appears to the right of the first gate you saw, replacing the switch that would close the first gate.
The one secret is the same:
- In the room before the fake exit, the left wall reveals a rocket launcher.