Inside the door under the golden key is a switch on the wall - too high to reach. Use the small corridor opposite the switch to get a floor up. Run/jump to hit the switch.

A door opens a bit further down the corridor. Work your way trough. In the end, you'll end up in a room with a Shambler. There is a hole in the floor, drop down and you can get to a red armour.

When you jump down from the Red Armour platform, you are on the same floor you used to get to the switch in #1. One of the holes in the floor is a deep well (clean the last hole out, please). A few feet down you'll find a ledge - and some company. Shoot the bit of wall, that has floor texture, and you open a goodie room.

Jump down to the next ledge, about halfway down. That's the secret.

One step furter down (splotch) this cage awaits. First, go past it and ride all the lifts to the top - else you won't be able to end the level. Get down again. Push the wall switch - one of the top beams lowers. Use it as a lift. Get into the cage, press button #2, and ride a floor down. Shoot the red wall switch to get out

Just follow the path from there: you'll end up with the gold key. Take it to the end of the yard, open the gold door and receive the fabulous award.

This is right inside the blue key door. Drop down trough the triangular hole. I guess you can find the way out on your own.