A weapon used during the Middle Ages in siege warfare. The Ballista was similar to a Giant Crossbow and worked by using tension: it was an invaluable Medieval siege attack weapon. The Ballista design was similar to a giant crossbow and worked by using tension: it was designed to aim huge wooden, iron clad, darts or arrows which were powered by twisted skeins of rope, hair, or sinew - the ballista design was based on a huge dart-throwing machine. The Ballista loosed heavy bolts, darts and spears along a flat trajectory. The force of the missiles launched from the Ballista was designed to have great penetration and were capable of skewering several of the enemy at one time! The word 'Ballista' is derived from the Greek word 'Ballistes' meaning to throw. In England siege weapons, including the Ballista, was also known as the Ingenium from the Latin word ingenium meaning ingenious device! Plural - ballistae. (from middle-ages.org)