Hang from the edge of the circular platform. Release many times to grab the handholds below until you reach the ledge. When you do that, you will hear the chime signaling the Treasure.

Grab the panel in front you and move to the right as much as you can. That way, you move the panel and the entire wall. This will reveal a secret alcove behind the panel. Go to the left, enter the alcove and pick up the Treasure (27/30).

This will reveal a secret alcove behind the panel. Go to the left, enter the alcove and pick up the Treasure (27/30).