Just before you enter the fortress, walk over to this spot. There is a submerged room underneath.

From the ledge over secret #1, go inside the door and down the stairs. Under the stairs is a secret room. Shoot the door to open.

This is right next to secret #2. Go up the stairs to the next corridor. Shoot the wooden door. Follow the ledge to it.

Step off the top step in the cosy little room with the blue key. You will land in the acid, right in front of a small secret room. Armour and a protection suit inside.

If you jump in the acid outside and double back a bit under the moving platforms, you'll find a Quad damage. Swim fast.

Reaching the silver door, don't open yet, but carry on along the corridor. Stand on the very edge of a drop to the floor below - opposite is a barred room with goodies.

Go back and look out across from the blue door. Shoot the target. Back to the now unbarred room, grab the stuff, and open the blue door...