Go to the other side of the ledge (to the right as you face the bridge) and jump up to grab the wooden handhold. Jump up once more to grab the second handhold and traverse to the left.

Jump back to grab the wooden ledge behind you and pull up. Step on the beam to the right and walk to the other side...

... until you reach the wooden platform. If you want to move faster, hang from the beam and traverse to the platform. Kill the two bats that attack and pick up the Treasure (3/28).