We're about to finish exploring the area
We're about to finish exploring the area, however we have to start the second elevator before we can continue. You can return to the rampart with the stormhawks throwing barrels and Elden Ring Runes find the balcony that faces the ocean. Peer down over the edge and you'll be able to see a platform below you can descend to. From here , you will be able to drop into a ledge along the castle wall or onto a destroyed tower which is where you'll find an Marred Wooden Shield.