Once you go through the first door, open the wall immediately north of you to find a chaingun and a backpack. (sector 26)
In the red rock cavern where the blue keycard is found, Revenants will come out of the west wall, as will multiple smaller monsters when you climb up the first step. Walk through this wall yourself to find a rocket launcher and a medikit. (sector 130)
In the room where you find the yellow keycard, the western pair of skull panels has two health bonuses in front of it. Push it, and return to the hallway you got there from to find that the wall has lowered below one of the northern monitors. Stand on it before it rises again, and walk through the monitors to obtain a soul sphere. (sector 202)
In the same hallway, one of the southern monitors is flickering. Shoot it, and then use the teleporter inside the yellow keycard room. Backtrack to the start of the level to find that one of the skull pillars has risen to reveal a teleporter. (sector 198) Enter it to be brought in front of the yellow key-locked door that the floor lowered in front of, and open it to find combat armor, a soul sphere, a megasphere, a BFG 9000, and some Arch-Viles (two on Hurt Me Plenty or lower, and three on Ultra-Violence or higher.
In the stone room where the red key is found, part of the northern wall is covered in far more green moss than the others. Push on it, and it will temporarily lower, carrying a berserk pack. (sector 200)
The back side of linedef 950 (the wall you push on to access secret #5) is untextured. As the ceiling of sector 200 is 32 units higher than the sector on the linedef's front side (sector 156,) if you stand on the lift and face outward as it rises, a Hall of Mirrors effect can briefly be seen before you are lowered again.