Getting to chest in Treviso

I'm getting a bit frustrated in Treviso trying to get to this part of a building in the north of the map. I end up in this courtyard with no way over, through, or around the building. I've run back and forth on different levels and around different buildings and can't see a way to get to it. I'm at 44th level, so pretty far into the game (I think) and am going to do the Unwanted guests quest once I finish exploring Treviso. Is it something simply not available yet, or at all? I saved Minrathos/Docktown, not Treviso in my playthrough if that makes a difference.