A ballista was a siege engine used by both the Broddring Empire and the Varden. Roughly resembling a giant crossbow, one was used to help the villagers of Carvahall escape from Teirm, causing serious damage to parts of the city. Ballistae were also mounted upon the walls of Borromeo Castle, in the event that a hostile dragon should try to attack the castle.
The ballista works on the same basic principle as a crossbow, i.e. it consists of a long beam with a nook into which the projectile is inserted. The rear end of the projectile rests against a rope held on two lengths of wood, attached to the main body by torsion springs of rope. In normal crossbows, the string would be drawn by the shooter manually. As ballista arms were far too powerful for anyone to pull, the string (or rope, in this case) was fastened via another rope to a ratcheted winch, which was used to pull the firing rope. Ballistae mostly had wheels for transportation. It should be noted that crossbows were developed by adapting ballistae to a smaller size, not the other way around. One difference between crossbows and ballistae is that crossbow bows were made of one piece of wood, while the ballista relied on two.
Ballistae were essentially oversized crossbows, as well as were accurate and powerful. The Roman civilization was known to make ballista the size of a small house. It required enormous strength to wind the string back, even with mechanisms such as the ratchet winder. Ballista fired large arrows or bolts, known as javelins, which could be lit on fire. A ballista could fire one or many javelins at a time, making them effective against dense infantry formations. Due to their great accuracy and range, ballistae were effective against ships. A flaming ballista javelin could take down a boat in a single hit.
Although ballistae were well suited at fighting infantry, ships, they were not useful at attacking static fortifications. Unless on fire, a ballista bolt would do very little to the stone fortifications of a castle or keep. Even if a ballista bolt was on fire, it would still cause relatively little damage unless it hit a critical area, like a stable or wheat store. For this reason, ballistae were usually used to defend castles, not to attack them. Boats also had ballistae mounted on them, though they were used to attack other boats, not buildings on shore.