When approaching the room with a T-shaped walkway over a blood pool, fall into the pool on the right side and pull down the blood fall. This will reveal a passageway (#1) containing a medikit, 4 shotgun shells and a berserk pack.
In the room with the yellow key, east of the stairs coming up from the blood pit is a support beam with a candle in front of it. Open it to reveal a skull switch and shoot it. A room will open to your right (#2) with a box of rockets, a medikit (and on hard skill levels a soul sphere) and three imps.
In the library, go up the stairs created by pressing a skull switch in the room. Pull down the bookcase with a candle in front of it for a green armor (#3).
After hitting the yellow switch, go into the bloodfall area guarded by arachnotrons. Press the red skull switch, then return to the library where some bookcases have opened up to reveal a plasma gun and two energy cells (#4).
After getting the red keycard, a small hallway to the left of the staircase opens up. Open the wall at the end of this hallway to be teleported to a blue armor and a backpack (#5).
From secret #5 walk through the door to open a hallway with a soul sphere (#6) guarded by two arch-viles.
After opening the red door, go through the left door. There is a gargoyle-faced switch in the middle on the right hand side. Hit this switch, then go back into the previous room. A new room has opened up (#7) containing a BFG9000.
Between the T-shaped walkway and the library is a hallway. Press the northern skull switch on the eastern wall to open all four switches, each one revealing a cache containing a lost soul and some items. Push the face in the northwestern alcove (behind the backpack) to open a door in the northeastern alcove. Pass this door for a alternative way to official secret # 6.