Once you kill everything that moves, walk up to the edge of the wooden platfrom near the grinding machine. Jump to grab the metal band on it and traverse to the left around the machine. Keep traversing until you reach a wooden beam.

Traverse along the beam and pull up at the end. Jump up to grab the ledge above and pull up. A giant spider will come up from below, so kill it.

Then smash the vase on the left and pick up the Treasure (18/28).

Use the Grapple to catch the metal ring on either of the two vertical planks. Walk around the other plank so the cable actually hugs it. Stand between the two planks and press the Interact (Y) button to pull the cable and...

... bring the planks together. When this happens, they will sink in the floor and a gate at the other side will open.