I think you need more variation in the foreground tiles, because as of now, there's only a few that I see. This is where decay really comes into place, out of the janitor's reach. Maybe have some tunnels dug by escaping prisoners/hungry dragons, attempted tunnels running into something impermeable, and simple smashed bricks. They have a lot of weight on them. And lastly, I'd add some stains, especially near the surface. For one, I don't see any toilets...