The Ballista was a Greek weapon developed for siege warfare. Many variations were created, but the concept remained the same: levers and torsion springs were the mechanics that ejected projectiles such as heavy spherical stones or darts. As time passed, the weapon was able to deploy objects that were destructive enough to level down buildings and fortifications with ease.
The Ballista is capable of ripping apart enemy buildings and reducing them into rubble. What makes them different from Rams is that not only can you attack the enemy Wall, but you have the choice to target the City Hall, Warehouse, and Farm as well. A good choice when attacking is to target the Barracks or Rally Point so they are not able to defend.