Castle builders considered the entrance to be the most vulnerable part of the castle, and they lavished attention on elaborate defenses for the portal. Muslim military engineers were especially adept at building complex gatehouses, and western crusaders learned from them and developed their own elaborate fortified gates. The portal itself had a heavy wooden door, often reinforced with metal, and one or more wood or metal grilles, called portcullises (port—gate, coulis—a sliding door), which could be dropped or slid into place from the upper chambers of the gatehouse. The passage through the gatehouse was also carefully designed—sharp turns prevented the use of a battering ram or a rush of troops, while holes in the vaults (“murder holes”) permitted guards to shoot or drop missiles on people below and also to pour water on any fires the attackers might build against the wooden doors. One of the finest gatehouses in the west was planned for Caernarfon Castle in Wales (see Chapter 3) where (if it had been finished) five doors and six portcullises, as well as turns and murder holes, defended the entrance. The gatehouse also had rooms for a permanent troop of guards. Since the castle was usually surrounded by some kind of ditch or moat, which might be either dry or filled with water, a bridge that could be raised or turned also defended the entrance. A raised drawbridge added its weight and thickness to the entrance portal. A small fortress called a barbican, built in front of the gatehouse, re inforced the effectiveness of the defense. Its towers and walls formed a trap for the unwanted and unwary. From the walls and towers of the barbican, defenders could fire down on attackers, turning their rush on the entrance into a murderous slaughter.