Ballista can destroy shields, barrels and chests, small obstacles, and can kill every human enemy with one shot.
For castle walls and towers - catapults and trebuchets can be used. But ballista is always enough to keep your enemies soldiers in fear. The one who will first take control over one will definitely gain significant advantage.
Excellent job! You might wanna consider adding some particle effect when the ballista is shot. Dust explosion or any highlight-effect showing the player more, he just has shot another projectile.