I would recommend looking up some aerial images of castles or watachtowers to see what they look like. That would likely give you some more ideas. Also, as far as making them "seem less 'flat'" it comes down to lighting. You can do a lot by making the lower areas darker and/or the higher points lighter. As far as I know there is no automatic filter that will do this for you. True, the drop shadow can help if you put it in the right direction and you aren't top dead center over the top of something.