The Ballista
The Ballista was a siege engine in the form of an extremely large bow, which shot huge and heavy arrows, or stones.  As its name implies, it was introduced to Europe by the Romans, although its roots were even older.  Originally developed in 399 BC. by the ancient Greeks at Syracuse, the ballista was in effect a giant crossbow, used for shooting large arrows or darts. It could peirce walls and small buildings, and was a powerful anti-personnel weapon, capable of transfixing several armoured men with a single dart, and its use continued long after their civilizations ceased to exist.
The precise design of ballistae was a triumph of ancient engineering and mathematics. The surviving manuals provide exact measurements and calculations for building ballistae of any size, and they were used and adapted by most Medieval armies.  Its amazing power was derived from the kinetic energy, or tortion, provided by the use of tightly twisted ropes called skeins.  A strong wooden frame carried the two skeins vertically, with the dart resting in a groove in the horizontal stock of the weapon. Two horizontal wooden arms passed through each skein and were linked by a strong bowstring. As the arms were pulled back, the rope was twisted to become a powerful spring. Like a large crossbow, the bowstring was pulled back by a winch and retained by a rotating trigger or nut.  The general range was about 400 yards when throwing a stone of about 60 pounds, but it could send huge arrows or sharpened logs even further distances.