Vasco

Ill try to put this the easiest way I can.

Press the square button while standing where you are to call the guard. "Sneak" by him by running in an extremely conspicuous circle along the wall and up the stairs.

Make a right at the first fork, run to the east side and call the guards to the north down toward where you are standing. Now run back west toward where you entered. At the first fork, press square to call the guards down, then run right. The Hawk Signet here will not react with your Crescent Stone, so you need to keep looking. Head east, make a left, then another left to find the Lion Signet. Use your Crescent Stone here to lit the path to the secret chamber. Proceed west to the fork and call the guard to the north down to where you
Are. Run back east to the opposite side, then up, and finally left to safely
Reach the hidden chamber. Approach the wall and press X to reveal the room. Thats it, Hope I Helped