During the 13th century, trebuchets were constructed for throwing heavy ammunition. They were made from hides, timber, lead, pig grease and rope. The basic parts of the siege engine are the base, the glide track, the support frame for the tower, the lever for the swing arm, a counterweight, the sling and the trigger.
The size of trebuchets during the Middle Ages varied. Some were able to launch projectiles weighing from 800 to 1,500 kilograms. The average range of such projectiles was probably around 300 meters. Various types of ammunition were used for trebuchets including stone missiles, beehives, clay balls filled with small stones, barrels of flaming tar and oil, dead diseased animals, and prisoners or spies. The siege weapon worked by transferring the momentum of a falling counterweight to a projectile via a long swing arm. The swing arm was attached to the frame of the siege engine at a pivot point and was joined by a large iron bar. This point was normally greased with pig grease or fat in order to reduce the amount of friction and achieve maximum range. Prior to the trebuchet counterweight design, the weapon was launched using only manpower. Men would line up behind the weapon and pull on a rope attached to the opposite end of the swing arm to provide the necessary force to hurl the projectile.