Close by where that door is that you can't open you should remember seeing the ground shake as you went towards the building with the bathing pools. Check around that area (look up especially) and you'll see it's rather unstable. Don't stand underneath it but look around the supports and see if your cursor doesn't change shape. That should give you enough to go on. P.S. the broken bridge is a blind. You don't use it but get to that area via a more roundabout way.