Ballista towers fire a huge spear-like projectile that does overwhelming damage to siege weapons. Although it takes a while to reload and cannot shoot units next to it, the ballista tower deals out massive damage with each shot.
Great against siege weapons
Cannot attack units in close proximity. Try grouping with Crossbow Towers.
Takes time to reload
Placing ballista towers near areas of wall that are vulnerable to siege weapons is a great way to utilize their power.