As in the Rochester siege, castle defenders did not sit idly by while an enemy was digging a sap. Typically, archers sprayed the areas around a tun- nel's front end, hoping to pick of some of the diggers as they entered and exited. h e defenders also learned to dig their own tunnels directly below those of the attackers, thereby causing the upper saps to collapse before they reached the walls. In addition, major improvements in castle design occurred after the start of the First Crusade in 1096. At the urgings of Pope Urban II, thousands of knights and other European soldiers journeyed to what are now Palestine and Israel. h eir objective was to free Jerusalem and other local sites that Christians viewed as sacred from the control of Muslim forces. During the expedition, the European i ghters saw many Middle Eastern fortresses up close. h oroughly impressed, they later carried numerous architectural concepts back to their homelands. "h e results," historian Frances Gies explains, "were an astonishing leap forward to massive, intricately designed fortresses of solid masonry."29 Among these construction ideas that European builders incorporated into new castles was the arrow loop, also called a murderess. It consisted of a vertical rectangular opening in a castle wall through which skilled bowmen could shoot arrows at besiegers outside. h ese archers were well protected by a clever feature. h e outside of a loop was very narrow, so most enemy bowmen were unable to shoot arrows into it. On the inside, however, the loop sharply widened, allowing the defending archers ample room to maneuver and i re. Another potent new design concept was machicolation. It consisted of a stone ledge or shelf that extended slightly outward from the top of the battlements. Usually the shelf was drilled with holes through which defenders dropped boiling oil or rocks onto attackers below. Also incorporated into new castles was the drawbridge. A wooden platform lying across the moat, it gave the defenders ready access to the front gate during peacetime. Yet the bridge could be pulled upward during a siege, making it extremely dii cult for the attackers to reach the gate and employ a battering ram to break through it.