At the southwestern-most end of the map is an outdoor blood-floored area surrounded by wooden walls, with a red brick structure in the center. There is a wooden staircase leading up to the north. At the top of the stairway is a door, with a small room behind it. In this small room is a skull switch and a window overlooking the blood pool. You'll need to run from the raised doorway (notice that it's a little higher than the actual floor of the room) onto the windowsill. This is really difficult to do (and may not even be the intended method) and will likely take several tries; straferunning helps. An alternative is to enter the lift that is activated by the skull switch and start running toward the window just when the lift starts to raise, then stop your movement just in the right moment, no straferunning needed. Once on the windowsill, you'll see an alcove with a soul sphere on it in the wall below. Drop down into it to get the secret.
Return to the small windowed room from secret #1, and flip the skull switch to a lower a lift behind you. Ride it up, and follow the path to the blood river. Run along the river onto the structure in the middle of this area. Go through the skull door, then turn 180 degrees and flip both skull switches, one on either side of the door you just came through. Drop back down into the blood and make your way to the north side of the structure and flip the skull switch. Step through the adjacent teleporter and run back into the central structure. Go through the skull door again and run through the leftmost window ahead of you to regain access to the rest of the level. Return almost all the way to the beginning of the level, to the hall with the scrolling spine texture high on the wall. At the west end of this wall a wall will have opened, granting you access to a secret room which held an Arch-Vile inside.
At the northern end of the map is a plus-shaped hallway with a pillar with red lights in it in the center. Down the eastern arm of the plus-shaped hallway are some stairs leading up to a window-lined room in which three platforms with Arch-Viles will rise upon entering. Once the three platforms have risen and the Arch-Viles killed, stand on one of them and run through a window. You'll land in a slime-floored outdoor area with some ammo.
Press the skull switch in the room mentioned in secret #3. This causes a stairway to raise at the north end of the plus-shaped hallway. At the top is a pair of alcoves with blood pouring from the ceiling, one on either side of the entryway. Go through the eastern bloody alcove (the bloodfalls can be walked through; they are not solid) and you will fall down into a bloody room with some armor.
Across from the other blood alcove mentioned above (the one leading to the blue skull key) is an arched window with a soul sphere inside. Step through the second alcove and continue a short ways, then turn around and run back out of the alcove and into the window. The "lip" of the alcove gives you an extra height boost, allowing you to reach the otherwise too-high window.
At the northeast end of the map is a short bridge across a blood river leading to the building with the yellow skull key inside. There is a small portion of falling blood to the west of this bridge. Step through the bloodfall to be teleported to a nearby ledge.
Inside the red-skull-locked green marble building at the southeast corner of the map, go down the hall to the right into the gray marble room with four pillars in it. Press either of the skull switches at the back of the room to lower a lift, then ride it up and follow the hall into a second gray marble room. There's a demon-faced door at the top of some stairs. Walk to the top of the stairs, but do not go through the door yet. Return to the first gray marble room, where a secret alcove with some green armor in it has opened.
After opening the yellow door inside the aforementioned green marble building (at the end of the level), ascend the flights of stairs to a window-lined, gray marble hallway. Note the bars blocking a glowing, hexagonal red tunnel to the right. Continue along the path, then up some stairs into the exit room. Upon reaching the landing of the stairs, the bars down below will open temporarily. Quickly run back through the bars before they lower again (it make take a few tries), then through the narrow, hexagonal hallway to a secret area. Inside is a soul sphere.