This strange thing is right inside the gold door. First step on the floor switch. Get on the lift, and ride it up. Shoot the red target, and a column rises from the floor. Now you can jump across.

The next 3 secrets are underground. Get to this room, and step on the floor switch. Negotiate an agreement wit the inhabitants (= Shoot them all!), then enter this room. One wall looks odd, compared to the others. Shoot it, a lift shows. Ride it up. On arrival, to your left is a acid pool. Jump in, there is secret goodies in there.

Balance/jump the narrow ledges. Right next to the entry door is this opening. as soon as you enter, it counts for a secret. Carry on...

...And you arrive at this corridor. Climb out on the roof beams. Jump them, until you arrive at this power cell - that's the actual secret.

This can't be a secret? It's the exit?

If you use the blue key and lower the bridge, only the exit will be open. But if you jump from stick to stick over the lava, another blue door shows itself, just to the left of the exit. This leads to the last secret level in the game: The Nameless City.