Near the start of the level you will come across a mostly green marble room, with a vine-lined walkway leading to the next area. There are two shallow alcoves with slaughtered Barons of Hell in on them to the left and right. Both of these walls open, leading to a security armor vest. Dropping down onto the ground from here will allow you to get some boxes of rockets at the cost of fighting a few Mancubi.
Same as secret #1, but on the other side of the room.
In the first large outdoor area (with the white-stoned floor), there is a teleporter that takes you to a ledge leading back to the previous, higher area with the red brick turrets. Instead of going through this teleporter, face it and open the wall 90 degrees to you left. Follow the small secret tunnel around and open the wall behind the teleporter, and go through it from this side. You'll be taken to a ledge at the far end of the outdoor area, from which a soul sphere is accessible.
After the outdoor area you will be in a very blue, maze-like cave. From the entrance, take the first right, then hug the left wall until you reach a portion of the wall with an opening near the floor. Open this wall to get some health and shotgun shells.
A bit farther on in the blue cave is another portion of wall that has an opening along the floor; open this one in the same way as secret #4 to get some more health and a few bulk cells.
Once inside the big green marble fortress, look to either side of the door and you'll see a big green monster-faced depression in the wall. Open either of these, both are secrets containing a single box of shells (and two Arch-Viles).
See secret #6.
Only a few yards north of both of the demon-faced walls from secrets #6 and 7 are two more depressions with the same face. These , too, can be opened, revealing a teleporter that leads to one of the parapets on the outer wall of the fortress. Both have an assload of imps, a little bit of ammo and a single medikit.
See secret #8.
In the courtyard a bit beyond secrets #6-9, go through the teleporter in the middle. You'll be taken to a ledge overlooking the courtyard, with three lion-faced walls in front of you. Press the use key on the center one, then drop back down to the ground. Go to the northeast corner of the courtyard, where a small secret area has opened. Inside is a megasphere.
After going through the blue door, go up the stairs, around the corner, and up another set of stairs straight ahead (not the one on the right yet). In front of you is a dead end with a tall strip of lights and a box of shells. On the left (west) side of the dead end hallway is a misaligned portion of the wood wall; lower this wall to get a soul sphere.
After passing through the red door, follow the walkway to the left and up some stairs. At the top, there are blue and black flags hanging on the wall. Notice that the wall between the two flags is misaligned, but can't be opened. Instead, you need to walk through it, though it at first appears that you can't do that either. This is because your passage is blocked by some explosive barrels behind the wall. Blindly shoot at this wall until you hear the barrels explode, at which point you can finally walk through. To open the back wall of the newly accessible alcove, pres the use key on either of the flags. Inside is another soul sphere, more security armor and a computer area map.
Near the end of the level is a big, lava-floored room with a bunch of pipes in the ceiling with lava flows coming out of them. Visible high up in a wall at the eastern end of the room is a seemingly unreachable alcove with yet another soul sphere inside. Drop down into the lava on the north side of this room, and make your way to the northeast corner (in a narrow passage between the wall and the walkway). There is a gray teleporter pad in the wall here that takes you up to the supercharge.