Follow the passage along until you get to the forbidden ritual with the pool of red water in the center. From here, you'll see the Moon key door. Shoot the symbol above the Moon key door, and the wall to your left will open up to reveal the exit to the secret level. The regular exit is behind the Moon key door. Just take the lift down to get to it.
Forgotten Catacombs Go around the corner, and activate the switch to call the lift down.
Retrace your steps, and go through the Skull key door. After you go through a metal door, you'll see a wooden bridge leading the Eye key door on your left. Underneath the wooden bridge is the Eye key, jump down, and grab the key. It shouldn't prove to be too difficult find your way back to the Eye key door. Go through it, across the wooden bridge, and down the ramp to your right.