Moats are sovereign in this game. Dig deep and wide enough and not only will your greydwarf problems be over, you'll put a big dent in the ability of trolls to harm you as well. And when system events such as Skeleton Surprise begin, most of them end up all but helpless when faced with a good deep moat -- skeletons will simply topple into the moat, where you can deal with them at your leisure. You can dig ridiculously fast and deep in this game. It's a gift. Use it.