At the start, go towards the east and go through the door. Pick up the rocket to reveal a larger library with baddies. On a bookcase in the east side of the room hit the switch. With your BFG blow the enemies behind hit or race towards the invulnerability sphere and get the yellow key (A). More Hell Knights will be released, and another one will when grab ammo in the back of the new room. Hit the switch on a new section that was lowered.
Exit the library, go east and touch the skulls to lower a wall with Imps (B). Pass it and go west towards a door at the end of the corridor. In the large room hit a switch in the northwest corner to lower a large chunk of floor. Go through the door and then the yellow door to its right (C). Climb the stairs towards the end and get the blue key (D). Jump towards a platform in the center of the room and will be sent to a similar room (E). Hit the switch in the east wall and shoot the north wall. Hit the switch there and go through the door by trying to avoid the Revenants and Mancubi. On the other side of the door, quickly open the center wall. Follow the corridor and lift platform towards the end and hit the wall behind the invulnerability sphere and go through it(F).
You will be sent to a small room. Hit the wall in front to return in the large where you were heading towards the yellow door before. Watch out for numerous Revenants in the room and go towards the blue door. Hit the switch and get red key in the midst of the Hell Knights (G). Go to the door beside the blue one and hit the first switch on the north side (H). The floor will lower. Go towards the narrow corridor pass the Arch-Vile and exit behind the red door.
The first secret can only be accessed for the first 30 seconds of the level, so time is of the essence. First, make your way around the right side of the slime pit ahead of you at the start. Cross the tiny walkway alongside the wall to get to the other side (or jump down from the walkway leading up to the rocket launcher in the center of the pit, or touch the skull symbol mentioned above to release the platform with imps, then cross this platform). At the far left of the wall in front of you is a door; go through and turn left. The brightly lit corner of this large room is the timed secret (I).
After picking up the yellow skull key in the library, wait until the bookcase you entered from lowers. Press the switch on the south side of the bookcase to open a secret alcove behind you (J). The switch inside is the only way to re-open the bookcase, letting you out.
Just north of the library is a corridor leading east. Step into the circle of light at the end of this corridor to open the wall in front of you, unleashing some Arachnotrons (K). The area inside counts as a secret.
North of the corridor from secret #3 is another just like it. Step into the circle of light around the glowing skull decoration to lower the wall between them. In the second corridor, step into the light surrounding the glowing eye decoration to open another monster closet that counts as a secret area (L). Inside are a blur sphere and light amps.
Return to the second large area (where secret #1 was) and go through the door to the right. Inside the next room are two blue skull switches (M). Press the one on the right to lower the floor in the opposite corner, revealing a secret tunnel leading to a teleporter (N).
The teleporter pad in secret #5 counts as a separate secret. However due to the teleporter only being one way, there is no way the player can get on the teleporter pad to trigger the secret credit. Hence 100% secrets can't be obtained on this map, unless via idclipcheating code.
After picking up the blue skull key, a wall opens up, revealing several Mancubi. The alcove they were hiding in — which contains a berserk pack, a soul sphere and a computer area map — counts as secret (O).
Step into the raised square in the center of the room containing the blue skull key to be teleported to a room identical to it. The alcove analogous to secret #7, which contains the same items, is also secret (P).
Secret 6 can't be reached to be triggered, making it impossible to get 100% secrets without cheating.
There are two Computer maps in this map (see secrets #7 or #8 above). In Vanilla Doom and some source ports, this makes it impossible to get 100% items, since the player can only pick one map per level.
The red key door by the exit is only 8 units wide, and the opposite-side's linedef is set to Line Type 1 (no key required). This allows the Arch-Vile to open the door (see Monsters open locked doors).
It is possible to get trapped on the ledge with the Pain Elementals and BFG9000 if the player uses the teleporter (secret #5) to get up there without having first ran across the trigger which open up the ledge (the trigger that opens the ledge is just to the right of the blue skull switch that controls access to the teleporter).
Sector 66 is not closed and one of its linedef is flipped so it causes a bleeding ceiling.
The platform holding the red key is lowered with a switch of type 71 (S1; Highest Neighbor Floor + 8). If the player does not lower all four platforms holding the Revenants surrounding it first, then the red key becomes inaccessible.