Open the door in front of your starting position, following the corridor to the right, and then the left. On making the second corner, a tripwire is activated. Make a 180 turn and run back to the starting room. The eastern wall is now open, leading to a supercharge.
Follow the instructions in Secret 1, but don't turn left following the corridor. Open the door, and immediately bear right. Push the candlelit wall to reveal a passageway outside leading to security armor, ammunition, and a rocket launcher which can be reached by pressing a switch to the east of it's plinth. Follow the valley counter-clockwise to reveal health bonuses and a berserk pack guarded by a revenant. Carefully run along the outcrops to reach it.
Collect the yellow keycard, and turn around. In front of you is a panel with a demonic illustration, press it and it will lower slowly. Step inside the wallspace to find a medikit, shotgun shells, and armor bonuses.
Collect the blue keycard, and turn around; there will be two switches either side of the corridor. Press the right switch, dropping you in from of an imp's cage. Swing around 180 degrees and press the switch before the platform raises again. This opens a wall at the end of the blue keycard hallway, revealing a chainsaw, health bonuses, armor bonuses, and stimpacks.
Return to the two switches seen at Secret 3. Now the far wall is open, hit the left switch and run to the computer map before the plinth pushes it back out of reach.
Through the blue door, a platform will lower unleashing a string of imps and hell knights, among other monsters. The first wall insignia to the left can be opened to reveal a passage outside. A switch in the northwest corner will lower the megaarmor plinth and activate a monster closet beyond, containing health bonuses.