Shoot the sun picture over this door. The wall with stained glass window opens.

Right inside the door with the sun picture (the same you shot to get secret #1) is a 'healing pool' in the floor. It counts as a secret, if you jump in.

Speaking as a mapper, nice to see that ID can screw up, too. Look at the underside of the 'sun' arch :)

This one is easy, but tricky to show: darkness. Inside the marked doorframe is a lift. Step onto it, and ride it halfway up - then step off and dive under the lift. And the red armour is yours.

Don't attempt to squeeze under the guillotine blade - it will leave marks. Go to the far-away wall. When you try to pick the ammo up, the walls left and right will drop down. Ride the left wall up again. After a couple of seconds, a chamber opens, containing a goodie and a slipgate to the other side of the guillotine.

This is a gold-plated bastard. Grab the blue key and the Protection. Roll a grenade into the hole under the slipgate. Then run over and stand on the grenade, and wait for it to go kaBoom!

This is dead against any survival manual i've ever seen. Yipe!