In the next hallway you'll meet a few bull demons and more zombies. The alcove the soldiers appear in holds the key to a secret area. The next room is quiet and calm when you enter. You just know it won't remain that way very long, so before activating the wall switch there, it's a good time to get to the first secret. As you move around to enter the lower area, more zombies will teleport in.

Stepping across the last step lowers a section of wall in that alcove you past before. Run back through the door to get there before it closes. The switch there opens a secret room next to where the green armor was. Make your way back to collect the rocket launcher. A few more zombies will teleport in. Take them out and return to the wall switch.

Activating this switch will open the nearby doors, releasing cacodemons. If you quickly make it back close to the door, they are pretty easy to bring down safely with the shotgun or rocket launcher. Beware though, when the first wave falls, more will teleport in. After this battle, the center column will lower to reveal the blue key.

Through the blue door are more bull demons and an apparent dead end. In the room near the end, you'll see a suit of blue armor, but you probably won't need it yet. Entering the room will bring in more teleporting soldiers. Entering even further into the room will cause the walls to lower, leaving you surrounded by imps. Again, this is an easy battle if you get back to the doorway and let them come to you. There is also a chaingun here, and another wall switch. This switch opens the bars near the blue door, allowing access to another hallway.

At the top of the short stairs, there is a section of wall to the left with a strange texture. Pressing use here will open a tiny secret room holding the alien laser, just through the next doorway. There is another set of stairs down near here, with another blue key door. This stairway is trapped however. When you head towards the door, you will instead be teleported into a crusher trap with a few bull demons for companionship. The two alcoves at the far end are also teleports. Both will move you to a safer ledge above this room. Imps and zombies will teleport in around the perimeter, so take them out and let the crusher do it's job on the demons below.

Yellow key:

Behind you in this upper alcove is the door out. This leads to another room guarded by more shotgun zombies. A lower area surrounding this one is filled with more imps. Quiet them all before proceeding. When you step into the central square of this room, a hole will open up with a switch to shoot at. Shooting this opens a door and allows a pair of hell knights to chase you around. The lower room where they came from has a few lost souls and an imp waiting. The yellow key is also here, temporarily. Activating the switch will transport the key to the center square of the room you were in before. When you collect it though, another group of lost souls will appear. The third and final secret area is also in this room. Using the auto-map, you can spot a section of wall that can be opened. Inside is a berserk pack.