Level 12: Altar of Pain: 2 Secrets

At the start of the level, turn right and go through the tunnel across from you. In this tunnel, examine the wall to your right until you find a hidden door, revealing a Soulsphere. Step into this room to get credit for a secret.

From the first secret, progress the level until you get to the Yellow Key. Once you get to the Yellow Key, pick it up and then RUN onto the platform across from you, which has a Soulsphere. You'll also notice that this platform is shaped like an arrow. This "arrow" is pointing towards a wall. Face this wall and RUN off the platform and you will go through this wall and into a hidden room. You will get credit for a secret when entering this room. Of course, in this room is a teleporter, which is the secret exit, leading to Level 30: The Lair.