Directly ahead of the starting point is a demonic pentagram. To the left is a switch to open it. Open the wall across the switch for a niche (sector 478) containing five armor bonuses.
As you exit the cave and cross the blood river in the west, you see a blue door in the northwest. Drop into the blood to its south and open the northwestern back wall. Follow the new path (sector 442) to pick up three health bonuses and encounter three spectres (demons on easy skill levels). Proceed and get the invisibility sphere (or save it for later), kill the three imps (two revenants and a hell knight on hard skills), grab the super shotgun and a bullet box. Note the five red brick pillars (two with skull switches), in this description numbered from west (#1) to east (#5). Push the switch on the pillar #2 to lower the pillar #3 to the floor level. Push the switch on pillar #1 two times to raise pillar #3. From pillar #4 step over pillar #3 (thus collecting the shell box) to the ledge in the southwest. Ascend the steps and run over the gap to the skull switch in the west. (From here you can reach the berserk pack on pillar #1.) Push the skull switch to open a wall in the north, releasing five imps, four demons and two spectres. In their alcove you find five health bonuses and two more red brick pillars with skull switches to control pillar #4. Push the eastern alcove pillar switch once to raise pillar #4 once. (Use the western alcove pillar switch once to lower pillar #4 to the floor level in case you are stuck.) After that, use the switch on pillar #2, then the one on pillar #1. If done right, pillars #3 to #5 should build a stair. Run over it to the east to jump on the platform with the before unreachable skull switch. Use it to lower it as well as the wall in front of you, letting you access a gargoyle switch that you have to use in order to access secret #10. (Here is a video that shows how to handle the pillars.)
Continue through the level until you reach the area where you see the blue skull key on a pillar about to raise out of reach. Follow the blood stream with stepping stones to the north, where an arch-vile (a baron of hell on easy skills) attacks you. Open the red brick wall to his east for a closet (sector 387) with a chainsaw and two health bonuses. As you enter it, the wall behind the chainsaw opens, releasing a cacodemon.
After entering the wooden door in the east of the stepping stones mentioned in secret #3 and following the passage behind, you see a plasma gun in front of you. (Use the skull switch on the eastern side of the wall south of the plasma gun to lower its plinth, so you can grab it.) From the plasma gun go straight to the west. Open the western wall south of the torch for a medikit and two bullet boxes (sector 182).
In the yellow skull key room, use the skull switch on the northern pillar's eastern side to open two entrances (north and south of the key) to the passage leading to the key. (This also opens some walls in the blue skull key area, releasing monsters, so be prepared for visitors.) Use the skull switch on the southern pillar's western side to temporarily open the wall behind the yellow skull key, allowing to access it through either entrance. Quickly enter the southern one, ascend the stairs, follow the bend to the north and take the western branch, opening a wall in the northwest and releasing an ambush. Run to the west, grabbing the yellow skull key and leave the room to the north, where you find a dead sergeant. From behind the now opened northern wall in the room west of you comes the ambush, including an arch-vile, so try to kill him first. After cleaning the area, enter the room north of the lowered wall where you see a green armor atop of a blood fall. The eastern path leads to a torch, lower the northern wall east of it. Ascend the stairs to get to the armor (sector 355).
From secret #5 descend the stairs to the north, but not to the east. Instead, open the western wall to access the walkway (sector 570) from where three imps attacked you before. Follow it to the south, collecting six health bonuses and drop down into the southern room to find some shotgun shells and a lion switch that you have to use in order to access secret #10. Go east to lower a wall and return to the ambush room.
Return to the place where you got the yellow skull key and wait for the wall east of you to lower. Push the skull switch on its front to lower the soul sphere's plinth (sector 124) in the west of the room.
Enter the yellow door; follow the cavern taking the left fork twice, ascending some stairs. As you arrive in the lava floor cave with the stepping stones, look to the southeast where you see a stone floor passage on the ground. Enter it to find six health bonuses and a radiation suit. (sector 374)
Leave secret #8 and walk over the lava floor to the west. Instead of ascending the stone floor stairs, enter the lava floor passage to the south of the stairs to find two stimpacks, a blue armor and a satyr switch (sector 601) that you have to use in order to access secret #10. Approaching it opens two the southern alcoves, each containing an imp or a chaingunner (skill dependent).
If not done yet, push the gargoyle switch in secret #2, the lion switch in secret #6 and the satyr switch in secret #9. Go to the cave entrance in the east of the BFG9000 you saw before when passing by. The corresponding stones have been lowered, allowing you to ascend the stairs leading to the BFG-9000 (sector 294).
After picking up the blue skull key, reenter the yellow door, follow the cavern taking the right fork. Enter the blue door, ahead of you is a platform with a medikit and two rocket boxes with a mancubus standing guard. Press the wall behind it to reveal three imps on a staircase (sector 434) littered with five health bonuses and a computer map at the end.