The trebuchet was a serious weapon of medieval castle warfare and siege. Using a large counterweight, it was able to hurl huge stones against castle walls and was surprisingly effective. This working, life-size model is found at Caerphilly Castle.
A trebuchet (occasionally referred to as a trebucket) is a medieval siege engine, a weapon employed either to smash masonry walls or to throw projectiles over them. It is sometimes called a "counterweight trebuchet" in order to distinguish it from an earlier weapon that has come to be called the "traction trebuchet." Sometimes the word trebuchet is used to refer ambiguously to either weapon, but originally it referred to the counterweighted weapon.
The counterweight trebuchet appeared in both Christian and Muslim lands around the Mediterranean in the twelfth century. It could fling three hundred pound (140 kg) projectiles at high speeds into an enemy fortification, quickly obliterating walls and defenses. Occasionally it was used to throw the bodies of people and animals who had died from various diseases including the black plague over castle walls, in an attempt to infect the people under siege. This is one of the first mentions of biological warfare in history. Trebuchets were far more accurate than other forms of medieval catapults.
I found another trebuchet at Château des Baux, France. The resolution is not the best, but you can see the counterweight-shaddow, enough to find the exact place.