In the giant courtyard east of where you begin the level, there are two bodies on the west side hanging from two sections of the wall. The northern one can be passed through. To get there, however, you must have one of the Arch-Vileslaunch you into that section of wall with their fire attack as you run toward it. If successful, you will find a compartment with a switch. (sector 177) Flipping the switch causes six giant portraits of Adolf Hitler to rise from the floor, four stone plates bearing Nazi flags to lower from the ceiling, a swastika to form in the center of the room, and 16 SS Nazis to start teleporting into the room. Although the switch can be activated from outside the compartment, you must Arch-Vile jump into the compartment to trigger this secret.
In the eastern, winding tunnel in the center of the map, when you reach the giant waterfall, climb to its east side, which has a section you can walk through. Do so to get a computer map. (sector 582)
In the room with the red skull, there is a soul sphere on a small ledge. At the higher floor of the room, head to the east side, align yourself with the demonic face in the south, and drop off the southern edge onto the ledge to get the soul sphere and trigger this secret. (sector 1177)