Find a hidden Soul Sphere on level 11, Circle of Death/'O'
of Destruction!:

From the main circular path, go east through the open
doorway which is flanked by a pair of green torches.

As you step through, turn right immediately, and look for a
stain on the floor next to another green torch.

The wall panel next to the stain is false.  You can walk
right through it to grab a Soul Sphere in a small niche.

PC Doom2 players can click here to access a demo that shows
how to acquire the hidden Soul Sphere on this level.