This lowers a ladder to the top of the left tower (15f), so select that for a close-up and keep clicking the image until you get the Handle. Move back to the ruined temple and select the ruined tower on the right (15e), click the device with the chains coming from it, and use the key to activate it.
Once you’ve got it, click the door once more to get another Key. Back at the crossroads, examine the stopped pendulum (13b), click the hand of it to reveal a door, then use the cursed key to open it and get the sixth Chest of Gold.
