Open the doors ahead, go through and hug the left wall to find a crate with a Shield Belt. Approach the doors - there's a lever on the left to open them. Go through, follow the path around the corner and push the lever to lower the drawbridge.

Cross over go along the cliff wall and go onto the wooden beam that slopes down into the chasm. Jump to the ledge below and carry on to the watery chamber. Dive under the near wooden post, swim through the passage into a small chamber and push the lever to call the lift. Take it up and hit the lever to open the main gates below.