1. In the starting room, there is a secret on the far left corner from where you enter. To open it, you'll have to push the display switch for the super potion, which shows up shortly before the teleporter, and then run all the way back. Be warned that there is an enemy inside.
2. You can reach the display room with all the health bonuses by searching the north wall for a secret door. Before you leave this secret, look for a ladder down to the waterway beneath the floor to get a mana potion; this is the most commonly missed item on this map.
3. After you take the teleporter, search the wall to your left for a secret door that leads to the containment cell for the monster. (While you're here, rocket jump through the ceiling and make your way to the colored corridor containing a switch, which is necessary to reach the second ring.)
03 In the large room where you eventually fight the stage boss, rocket jump up onto the central structure and then jump up to the nukage containment pillar. The pillar can be entered from the north side to find a ring.
04 If you look at the ceiling of the same room, you might notice a hole in the grating, but the one near the nukage pillar is probably too high to reach. There's a similar hole in the balcony overlooking the room, right before the final few zombies and the trigger line for the boss. Rocket jump through this hole. Over the eastern side of the boss room is a path leading to a switch, but be advised you have to rocket jump to get back out. Now go south from the hole and into the colored passage to the east, where there's another switch overlooking the ring itself. If you already flipped the hidden switch in secret #3, return to the room that had the imps and female zombies and collect your ring.