I've seen it done before, but not since the buff to shard of patience to give nonwater creatures hp. Most people seem to prefer doing it that way, instead, though there are pros and cons to each. I don't think I'd use a gravity pendulum though; I'd stick to all towers, as you won't need much entropy quanta at all, and it will just take away from your gravity quanta. (it will provide some marginal earthquake protection, but not enough to bother with, I think)