Ride up the lift mentioned in non-official secret #3, go north and drop down into the pit. While the lion switches on the pillar lower the pit's floor to the eastern passage, use the gargoyle switches to raise the floor step by step until you reach the western passage and enter it. In the room behind (at the northwest of the map), climb up the stairway and approach the switch. An elevator (sector 497) will lower to the east, which counts as a secret. The elevator leads to a room with five chaingunners, four bullet boxes, five shell boxes, three boxes of rockets and four medikits.
Return to the pit mentioned in secret #1 and use either lion switch to lower the floor. Follow the eastern passage into the toxic sludge room (in the northeast of the map). Run up to the wall with the skull on it (sector 520) and open it to find a megasphere. After ascending the newly-revealed stairs in the room west of the toxic sludge room, the access to it is blocked, so you have to get the megasphere before.
In the courtyard with the four berserk packs, enter the hallway right east of the northeastern berserk. Open the portion of the southern wall lit by the lamp in the northern wall to reveal a balcony with a blue armor, two stimpacks and a super shotgun.
After using the teleporter (this works only once) at the end of the hallway surrounding the patio with the yellow skull key and crossing the blood pit, you enter a passage. Open the portion of the southern wall lit by the torch in the northern wall to reveal a secret passage. After killing the spectre upstairs, you find a plasma gun in the south. Approach it to ride a lift down to two imps. Take care about them, push the gargyole switch at the western wall and ride the lift back up. Quickly enter the opened wall east of the plasma gun to reach the soul sphere you saw before from the blood pit. After using the white teleporter in the room after the non-secret passage and returning to this room later, the non-secret passage cannot be accessed any more. So, in order to obtain 100% of the items, you have to collect the soul sphere before teleporting.
This one is made for coop-mode: After entering the red door you reach a room with eight green torches. In single-player mode, you have to push the gargyole switch in the northeast of this room to lower the lift in the west. In multiplayer mode however, this switch is blocked by a ninth green torch. So player #A has to enter the lit square in the southwest while player #B operates the lion switches on the southeastern wall, pushing the western switch twice to raise the square. (The eastern switch lowers it to floor level.) Player #B then has to wait in front of and southeast of the room's northwestern platform before player #A shoots at the red recess in the western wall. This raises a switch under the feet of player #B, lifting him up to the platform where a megasphere is on. (This switch has to be used to lower the western lift.) In single-player mode, stand next to the northeastern switch, use the super shotgun to shoot at the red recess west of the lit square. You propably need several tries until one of the pellets eventually hits the target. Quickly run over on the switch in front of the platform when it starts raising. Using a source port that allows vertical aiming makes this much easier of course.
Routes and tricks
In single-player gameplay, under UV speed, NM speed, and UV pacifist rules, the map is extremely easy to finish. Going clockwise around the building you start in not only lets you access the red skull key (which can be grabbed without lowering the platform it is on,) but also opens a shortcut into the area with the berserk packs, which allows you to reach the red key door, and consequently, the exit, very early on. This makes the map completable in less than 30 seconds.
On the other hand, in cooperative multiplayer, the walkover linedef opening the shortcut, as well as the quickest way to the red skull key, are blocked off by green torches. This is intended to force you and your team to travel through much more of the level in order to complete it. However, two tricks devised by Kristian Ronge, the current UV pacifist record-holder on this level, allow the map to be completed through the single-player path even with the torch barrier. The first is performed in the starting area, by moving carefully around the pile of skulls, making sure not to fall into the blood moat, allowing you to obtain the red skull key in that location. The second takes advantage of the ability to trigger linedefs from any height, by opening the blood fountain in the first room, standing at its southwest or southeast sides within activation range of the southern switch, and flipping it to open the shortcuts. From there, the route is the same as in single-player. The tricks are demonstrated in Ronge's 00:49 coop speedrun () at the Doom Speed Demos Archive, which requires ZDoom 1.22 and a command line argument to initiate a faked cooperative multiplayer game.