Get the rocket launcher and get down into this room. Going out on the second slope, there is a yellow light in the side of the ledge. Shoot it, and some stairs arrive.

Hope you don't suffer from vertigo. Jump down on this corner, a door wil open to a very rich room of goodies. Enjoy.

Shoot the four lights in the Secret #2 goodie room. Another room opens, with yet another goodie.

Nice, isn't it?

Ride the lift up after surfing the third moving platform. Dispose of the grumpy fellas at the top. One of big, clunky celing lights has a different colour. Shoot it. You get something you are going to need badly.

Real soon. Watch your step.

When you finished the Shambler Dance (remember the Thunderbolt), don't use the slipgate, but get under the stairs. The pipe in the lava is the entrance to the secret level "The Haunted Halls".