That's all true. The main point I was making is that the ballista isn't very good at destroying fortifications, it's main use was as an ancient sniper rifle. Keep in mind though you aren't just talking about a bullet shooting someone in the head either, it would skewer it's target with a 6 foot bolt, and would cut through armor like butter, and it could travel through multiple targets in a formation and thus skewer a line of soldiers, makeing an infantry kabob. Anyway though, what I was saying is it wasn't used like a treb or later cannons that would bust down walls, and it didn't inflict mass causulties. But as you say in a siege having a psycologically terrifying sniper rifle is probably pretty useful, and it's also really useful in field battles, as it can scare the . .. .. .. . out of infantry, and wreck their formations (how would it feel to stay in a nice square formation as every few minutes a bolt comes through and skewers one or two peopel?, I mean in the overall line, that's not doing much damage, but it's effect on morale would be devestating).