To be effective, the war machines needed level ground facing the castle walls. So castle builders positioned their buildings on cliffs or surrounded them with natural defenses or ditches. Many powerful castles had water-filled moats that were as wide as lakes or ponds. The attacking army had to begin its siege by filling the ditch or moat, perhaps breaking dams or diverting streams to do so. Then a causeway had to be built over which battering rams and siege towers could be rolled into place. A battering ram—a huge metal-tipped pole hung in a sling and protected by a roof—might be so large that it required a hundred men to swing it against a wall or tower (Document 26). Small rams could be operated by a dozen men and used in confined spaces such as gatehouses. As the ram pounded the wall, the defenders tried to absorb the shock by hanging bundles of wool or straw in front of the wall. The defenders also tried to catch the ram with grappling hooks and lift it into a vertical position rendering it useless. Less dramatic was the process of sapping, in which the attackers attempted to bore through the walls rather than batter them down. The men operating the equipment were vulnerable to missiles, fire, or hot pitch thrown at them by the castle’s defenders, so they worked under a moveable shed (penthouse) whose roof was covered with earth and hides. This shield was called a “turtle” because of its shell or a “cat” because of its sneaky approach.