To get to this mission's first secret area, walk straight to the right, into the water, you should see a entrance in the wall oppisite the castle, almost exactly where you land. Swim thorugh it and get the 100 health bonus, and nails. Swim back out of the opening and head right to a flight of steps. You'll need to fight some knights off, so you may want to return to this secret later.
Enter the castle ahead, kill all the knights you meet. As you go down the corridor avoid the pools of light on the floor, as they trigger nail-traps. Kill all the knights at the inter-section and collect the rocket launcher. There is a ogre above you to kill, then head left and kill the ogre waiting in the hallway. Continue forward, beware of the falling spike, before entering the room pause and try to kill the knights, steping onto the floor activates another wall trap. When you are ready run to the room to the right.. Shoot the here, and continue around the walkway. Pass through the first bar, hit the floor button, and try to pass through the other bars, if you get knocked off you will have to fight some fiends, and dodge a nail trap to return to the start of the walkway. Once past the moving bars, kill the knights by the stairs. Continue up the stairs, and be prepared for the scrag and ogre at the top. Step on another floor button to release another knight. Kill it and grab the silver key. Drop down through the floor. You are back to the main intersection, and a ogre maybe waiting for you. Run, and enter the silver key door. Push the button, turn around and get on the lift. Drop back to the intersection and head up the right fork.
Kill the knights and ogres you meet, and grab the super nail gun. Jump against the torch above the nail gun, if you do it correctly, you'll open a secret the other side of the pillar containing the yellow armour. Continue up the stairs, where you'll get to meet yet more knights. Don't go through the portal, instead step around to the back of it, and enter.
Run along the ledge from the fiend and scrag, to the quad damage. Powered up, kill them and return through the portal, grab the gold key and take on the shambler. Exit the room, and kill the knights and ogres. There is a secret of some yellow armour on the landing which contained the upper ogre. To get it, you need to jump from one set of stairs to the other wall, walk down the wall and then jump to the secret. Now return to the gold door. Open it and drop down the shaft to the exit room. Before going through the exit, turn around and shoot at the wall directly behind you. This reveals the last secret, some green armour.