Turn left and and notice the Treasure in the far right alcove. This is where you will go now.

Facing the gate hang from the edge on the left and shimmy left until you cannot go any farther. Jump to the left to land on the lower ledge and follow it to the other side.

You will see a pillar with small handholds on the left. If you pass the pillar, a green naga will appear from behind, so draw weapons and kill it. Three spiders will also join you, so take care of them too. Now climb the handholds on the pillar.

Jump up to grab the narrow ledge above...

... and traverse to the left around the two corners.

Jump to grab the ledge on the left and drop to the lower ledge on the right. Draw weapons and kill the red naga that attacks.

Hang from the edge of the ledge and traverse to the right past the wall. Pull into the alcove and pick up the Treasure (13/30).