The Trebuchet
The Trebuchet, as its french name might imply, was introduced to England by Louis of France in the 13th century.
The longer end of the arm, called the VERGE, was winched down to ground level against the pull of the counterweight by a system of winches.  This end carried the projectile, in either a spoon or sling which, when released, flung upwards with considerable velocity, hurling its projectile in a high trajectory to a range which could be adjusted by a sliding counterweight on the longer arm.
Working models of the trebuchet have been built in recent years, and used for flinging everthing from pumkins to pianos.  Tests have shown, that the range of the machine appears to have been about 500 yards, with deadly accuracy. It is not hard to see how much damage one of these could inflict, and indeed the walls of many surviving castles and castle ruins show the marks and damage caused in the past by use of these amazing machines..
The basic design was a long arm, pivoting on an axle at the summit of a high frame.  The shorter end of the long arm was weighted down with enormous containers, baskets, or buckets, which were loaded with earth, stones and rubble, and occasionally even lead bricks or weights, which acted as a counterweight to the projectile intended to be used against the target. 
Cut stones were sometimes used as a projectile, weighing as much as 300 pounds!  The standard weight however, was 60 to 100 pounds.  Then of these stones a day could be produced by a single stone cutter.  Stones found at Pevensey Castle weighed in at 240 pounds. Other projectiles known to have been thrown by the trebuchet included paving stones, dead horses, pots of quicklime, and even junk and scraps of iron.
It was a large siege warfare weapon used to break down the walls of castles. It was the successor of the Catapult, which itself had been developed from the Ballista, but it could take heavier rocks and projectiles, and fling them farther, with more accuracy.