You can use these to climb down below the bridge on the bottom-most handhold will be the Treasure.
When you do, go back onto the walkway and face the second tower to the right of the end of the bridge will be white handholds. It is recommended that you clear the second tower out of enemies first, getting at least to the point where the snipers aim inside the second tower. This Treasure is located after Nate fails his stealthy entrance and enemies come at you from a second tower and across a walkway. Use the handholds to climb up the wall between them to find the Treasure on one of the red handholds in the middle. To your right will be a wall with two doorways on either end. Head to the machine gun turret and face the direction it’s pointing. This Treasure is much easier to find than the last one. Head to the furthest edge and swing out on it, then drop down onto the red beam and shuffle out to the end of it, where the Treasure is waiting for you.
On the right-hand side will be an exposed bonfire area. Head back to the red gateway that was up the stone stairs and face the way forwards. The next Treasure is located at the start of the first major battle area, however you’ll probably want to wait until after you’ve eliminated all the enemies and completed the machine gun turret clearout before attempting it. Head to the gate, shoot the Treasure down, and claim your prize. You may have spotted it as you were sneaking along the cliff: it’s underneath a beam on top of the entry gate where the truck rolled on. The first Treasure of this lengthy Chapter is found right in the opening area.