The devastating power is best shown by King Edward's Welsh campaign, in which he built a series of castles to surround the Welsh prince's stronghold. Caerphilly castle has man-made lakes around it- large enough to keep trebuchets away. The presence of the lakes shows how much the trebuchet was feared. Enough to dig lakes over 200 yards wide around the castle... Untill the cannon, the trebuchet was the dominant weapon. The trebuchet relied on gravity to throw projectiles- stones, unliked messangers, and dead animals. It also used leverage and a sling for greater efficiency. The physics behind this weapon are simple in concept, but complex in detail.