You will reach a part where you need to turn off some running water through a water tunnel. When you get to the other end of the tunnel, there will be three thugs. After you take out the thugs, go to the ceiling. You will see a door. Enter it and flick the switch. Then, go back through the tunnel that led to the room. Instead of jumping across the water-filled cage on the floor directly under the tunnel entrance, go into it. You will see an open door which leads to a secret room.