Jump down from the point of the picture. Everything will go dark. Get to the center column and keep close to it going around it to the left. This secret is... not very secret, really.

Cage with 3 big guys in bad temper. Walk around it to the left. Everything is dark. You wonder briefly if their electric utility has been privatized. In the dark, there is a crack of light under one of the walls. Tap it for major goodie.

This one is... different. Each and every red-key bar holds _two_ switches! One on the front, another when you walk over the spot on the floor where the bar rested. Each triggers a door to a secret room with a big goodie. I haven't seen this use of triggers before.

This level holds the exit to E5M9: Realm of Iblis - the secret level.

Finish everything as you normally would... and then you need to go back to the 'maze' room with the tower in the top. At the very last platform before the jump to the hillside (the one directly south of the tower on the Automap) there is a very small crack in the cave wall, with a eye-on-a-stick switch. You have to climb on the edge of the brickwork to get the right height to shoot it.

Take the teleporter up to the cave; a new room has opened. From here you will be beamed to Realm of Iblis.