hese and other improvements in castle defenses made it considerably harder for an enemy force to capture a large stone castle. Still, many medieval European fortresses did fall, mainly because the besiegers had a wide array of lethal military tools at their disposal. he largest, most imposing, and by far most costly was the siege tower. Because it was very expensive and time-consuming to move over long distances, it was nearly always constructed from timber and other materials lying close to the castle. Hindley describes these formidable contraptions, frequently referred to as “belfries,” and tells how they carried soldiers up to and over a fortress’s walls: At their most elaborate, such towers comprised a wooden structure in several tiers, hung with soaked animal hides as protection from ire-arrows and with ladders going up inside the structure to the top platform. here might be a drawbridge, held in the vertical and afording protection to the troops waiting for the assault. As soon as [those soldiers were] assembled, the bridge was swung down [onto the top of the wall] and the troops forced themselves across as best they could. Ideally, before the charge, bowmen in the tower would sweep the enemy wall to clear it of defenders. [Such a tower was] mounted on a wheeled base so that it could be rolled against the enemy fortress on the orders of the commander.30 he larger belfries not only made it possible for soldiers to get inside a fortress, they were also capable of causing a huge amount of damage beforehand. A tower often held archers and slingers, along with catapults, all of which propelled deadly missiles over the walls as it approached. A giant siege tower employed against Kenilworth Castle in west-central England in 1266 contained more than two hundred soldiers and eleven catapults. As a belfry approached a wall during a siege, the defenders sprang into action. Knowing the device was made chiely of wood and other lammable materials, they shot ire-arrows in hope of burning it down. h ey also unleashed torrents of arrows and rocks on attackers who tried to deposit debris in the moat so that the tower could cross it to get next to the wall. Besiegers also routinely used artillery engines, machines that i red rocks and other projectiles at or over the walls. An example is the catapults that were sometimes lodged inside the belfries. Much larger catapults were typically arrayed outside a besieged castle to aid in the attack. Another destructive artillery engine employed in many sieges, the ballista, was an oversize crossbow that discharged spears and/or giant arrows. More deadly still was the trebuchet, which bore numerous nicknames, including “God’s stone thrower,” “war wolf,” and “bad neighbor.” h e device featured an enormous balance beam divided into a short arm and a long arm. University of California scholar Paul E. Chevedden explains how it worked. “At the end of the longer arm,” he says, was a sling for hurling the missile, and at the end of the shorter one pulling ropes were attached, or, in later versions, a counterweight. To launch a projectile, the short arm, positioned aloft, was pulled downward by traction or gravity or by a combination of both forces. h e impetus applied to the beam propelled the throwing arm of the machine upward and caused the missile to be hurled from the sling.31 Trebuchets, which came in a variety of sizes, possessed remarkable power. A large one could l ing a projectile weighing 220 pounds (100 kg) more than 1,300 feet (400 m)—about a quarter of a mile. One eyewitness account by an English priest describes its use in the siege of Acre (in what is now Israel) by a Crusader army: “It shot with such force, and its blows were so ef ective, that no material or substance could withstand the unbearable impact without damage, no matter how solid or well-built it was.”32 Not to be outdone, the defenders of some castles countered the use of such artillery by installing their own artillery inside or atop the walls. In fact, it became customary to hurl the same rocks an enemy had thrown right back at him. And so it went. As each new destructive device or idea incited the invention of an equally hurtful countermeasure, medieval siege warfare’s relentless arms race plodded on and on.