Ballista

The ballista (Latin, from Greek βαλλίστρα - ballistra and that from - βάλλω ballō, "throw"), plural ballistae, was an ancient missile weapon which launched a large projectile at a distant target.

Usage
Developed from earlier Greek weapons, it relied upon different mechanics, using two levers with torsion springs instead of a prod, the springs consisting of several loops of twisted skeins. Early versions ejected heavy darts or spherical stone projectiles of various sizes for siege warfare. It developed into a smaller sniper weapon, the Scorpio, and possibly the polybolos.

Role in Astro Knights
A ballista is seen on the Castle. It was used by the Royal Guards during the Battle of Arturus.To gain access to the Princess's room,you must attach the rope to the balista missile and shoot it until the missile hits the door to the Princess's room.Then walk along the rope like a bridge to reach the door.