Level 20: The Breakdown: 3 Secrets

When you start Level 20: The Breakdown, progress until you acquire the Blue Key and reach the Blue Door. Hit the switch in front of the Blue Door to open it. In the subsequent hallway, proceed all the way to the right until you encounter a wall. To your left, you will see a set of red bars. This wall appears slightly misaligned; examine it to reveal a hidden door. Enter this hidden room to find a Soulsphere and gain credit for the first secret.

From secret #1, turn around and return to the opposite side of the hall. Jump down into the large room to your right. After defeating the Nightmare Imps and Hell Knights, two staircases will rise, leading back to the previous hallway. Before ascending, hit the switch, and you will hear a door open. Turn around and take the staircase to your right. Enter the small, newly revealed room to collect the MegaArmor. If the door closes, descend the stairs, hit the switch again, and retry. After obtaining the MegaArmor, turn back and go down the stairs. A new room will be visible on the other side of this area. Enter it to find a Backpack and Rockets, and gain credit for the second secret.

From secret #2, continue through the level until you reach the red bars (after obtaining the Red Key). Open the Red Bars and activate the switch on the other side. Turn around and ascend the newly revealed stairs. At the top, follow the hallway until you exit outside into the maze area. Take the first path to your right and follow it to a staircase on your right. Go up these stairs. At the top, turn right and follow the path to a switch. Activate this switch. Turn around and descend the stairs you just used. At the bottom, turn right and follow the path to a four-way intersection. Take the right path to another four-way intersection. Go right, then ascend the stairs at this intersection. At the top, open the door at the end of the path and proceed through it. Hit the switch in this room, then turn around and go back down the stairs. At the intersection, notice two paths ahead: one with a small tunnel (on your right) containing a Medikit, and another with a staircase (on your left). First, go up the stairs and activate the switch at the top; you should hear a lift dropping. Immediately run back down the stairs and enter the tunnel with the Medikit. If you were quick enough, a room will have opened to your left within this tunnel, containing a BFG. Enter this room to get credit for the third secret. If you miss the timing, return to the stairs, ascend, activate the switch again, and try to reach the room faster. Strafing while running may help.