The ballista is an ancient missile launcher designed to hurl long arrows or heavy balls. The largest could accurately hurl 60-pound (27-kg) weights up to about 500 yards (450 m). The Greek version was basically a huge crossbow, while the Roman ballista was powered by torsion and used two separate arms joined at their ends by the cord that propelled the missile. Once the Roman Empire declined, so too did the ballista—it was too challenging and expensive to build. Which weapons took its place? Discuss