In Episode 2 Map 7 (secret level), shoot all of the gargoyle faces once. They will bleed. Then, go to the long, flashing corridor where you encounter two Hell Knights in alcoves. Shoot the wall of the second alcove to reveal a hidden room containing the initials of Tim Willits and Teresa Chasar, along with a raven symbol for the raven levels...
Nine Inch Nails logo
Look closely at the nail gun ammunition boxes to see the "NIN" logo for Nine Inch Nails, the band that did the music for the game.
Well Of Wishes secret
This is a secret extra for the Crypt Of Decay (Episode 2 Map 3). After the spiked hall, go forward, turn backwards, and then look to your right. There is a room there where you must take a leap of faith to cross. Get the quad and keep going towards the wall until you reach a hall. Enter it and go down and swim. The Dope Fish is at this location....