Oh yes. Let's forget that you'd need a horse to drag the thing around and the fact that a human would take at least twelve seconds to reload the bloody thing, and he'd need the help of a huge winch to do it. And that the bolts weigh at least a pound each. And that's only for the smallest types of ballista. Lances... meh. Same thing as a spear, only you use them from horseback. Unless we're talking about jousting lances, in which case you HAVE to use them from horseback, and they're designed to shatter on impact. Look, everything is possible in Runescape. A small dagger takes as much one space as a large Full Plate. Make Ballista like Cannon-like then, if that suits you.