This level occupies the map slot MAP25. For other maps which occupy this slot, see Category:MAP25.
Other points of interest
After following the blood river (see secret #4) and using the teleporter at its end, you are in a section of the map similar to the one you entered through the yellow door. You still can use the teleporter in the room similar to secret #3 to get back to the main section of the map. However leaving the semicircular stone floor around the pit where the rocket launcher is in to the surrounding stairs in the north lowers some bars that block the access to that teleporter. You have to fight two monsters (revenants on easy, arch-viles on medium and hard skill levels) that teleport on the pillars to the north while waiting for the bars blocking the exit to open. In single player, nothing but a rocket launcher, five rockets and a medikit in this remaining part of the map can be collected. So register any secret and grab any item in the rest of the map before leaving that stone floor. (In multiplayer, the bars can be opened again by a player that approaches those from the south of the stone wall with the demon face and pentagram on it. Also there are more items in multiplayer.)
From the map start go north and enter the right (eastern) tunnel. Walk over the step to your right and shoot at the demon faced wall downstairs. Push the gargyole switch that opens to your right and step on the lift. After riding it up, walk to the north over the gaps and ascend some stairs with the bend to the west. Kill the imp and step on the stair (sector 241) behind the berserk pack, where you find a shotgun, a medikit and a shortcut to the yellow skull key.
From the yellow skull key drop down into the blood pit, push the lion switch in the west, ride up the eastern lift and exit it. If not done yet, ascend the bloody stairs to the east and use switch on the wooden part of the wall to your right. (Its counterpart on the southern side has the same function, so you need to use only one of those.) Return to the map start and enter the cave in the west. Ascend the stairs and at the blood pool climb up the next stairs to the northwest. As you exit the cave, you hear a sound like a lowering lift behind you. Wait until you hear it rising again and turn around. Run downstairs, across the blood pool and towards the blood fall to be teleported to the top of another blood fall (sector 171), where you can pick up a soul sphere.
Ascend the stairs behind the yellow door and go north. At the fork in front of the stone wall with the demon face and pentagram on it, take the left (western) path. Open the westernmost, slightly recessed bookshelf to enter a passage (sector 792) with two health bonuses.
After lowering (or picking up) the red skull key (not the one that lowered into the floor as you approached it, but that one in the south of a rocket launcher), return to secret #3. Use the now opened teleporter and push the gargoyle switch to lower the bridge and cross it. On the eastern side, enter either tunnel to reach a circular passage (sector 445) with six rockets and a box of rockets. Registering this secret removes the bars to its east that block the secret's access from the area east of the red doors. It is recommended to do so to have the possibility to instantly retreat from that area, since once you passed either red door you cannot return into the hallway again until you followed the blood river to the east for a while to lower the steps (Linedef 2258, Action: 37 - W1 Floor Lower to Lowest Floor (changes texture), Tag 23) blocking the red doors from this side.
In the area east of the red doors, ascend the steps south of the blood river and enter the path to the east tagged with three rockets. Eventually you reach a box of bullets (in front of a green armor), enter the fake wall to its east to find a computer area map, seven armor bonuses and a partial invisibility. Walk behind the second armor bonus to register the secret (sector 496).
Aquiring the key the normal way (dropping down on it from the north) forces the player to proceed further to the south, thus first triggering the bars in the southeast of the bloodpit and the (closed by default) accesses of the pit's exit lifts to close (Linedef 1504, Action: 110 - W1 Door Close (fast), Tag 3) and then, 8 map units southward, while crossing the edge of the key's alcove (Linedef 895, Action: 38 - W1 Floor Lower to Lowest Floor, Tag 2) trigger the blood pit to lower, enabling access to the switch (Linedef 1522, Action: 112 - S1 Door Open Stay (fast), Tag 3) that opens the pit exit lifts accesses and the bars.
The usage of the secret #1 shortcut enables the player to pick up the key (that is close to the edge) without triggering any of the lines (- leaving the pit is easy as long as it is not lowered).
But it is also possible to step in the key's alcove, crossing the southernmost trigger lowering the blood pit where the player will have to drop down, not triggering the bars and pit exit lift controls to close (since that linedef is 8 map units northward), then falling into the pit and be forced to push the only once usable switch down there to open the exit lift access. After using a lift to exit the pit, reentering the key's alcove to grab the remaining shells finally triggers the lift access as well as the bars in the southeast of the pit to close for good, keeping the player trapped.