| When you enter the dungeon using elevator, turn left. There's a secret room you can eneter by pressing a loose brick on the wall. I think it's on left wall, near the gate where you can enter the water. Chceck the wall.
However you don't need that. You can swim underwater to get to the rooms. After you find the armor you're looking for, you can leave this castle using castle gates. | 
Broken lever in Fathien's Tomb 
Linear Mode
