As they captured each territory, William and his men secured their camps with simple earth and timber defenses characteristic of northern Europe (Documents 2 and 3). These wooden structures have disintegrated, but sometimes their earthen mounds survive as rolling hills or picturesque elements in the landscape. Timber castles were especially useful to a warrior king like William the Conqueror who moved rapidly to bring new territories under control. The earth and timber castles could be built quickly, since the newly moved earth did not have to support great weight. Such structures were also relatively cheap because they required no specialized masons and expensive stone. Since the timber building tradition was widespread both in the British Isles and on the continent, the carpenters knew the building techniques. These castles were essentially towers and stockades; they provided garrison headquarters as well as residences. William had learned the value, as well as the technique, of building and using castles at home in Normandy, where castles and siege warfare had been developed in the ninth and tenth centuries. The strong rule of Charlemagne had given the people of Europe some sense of security, so that Charlemagne’s grandson, Charles the Bald, even issued a prohibition in 864 against the building of private fortifications. But intensified Viking raids along the coast of France in the later years of the ninth century forced the people to insist on permission to defend themselves and to fortify their homes. By 869, Charles the Bald rescinded his edict, and the landholders began building walls around their homesteads again. Relative peace returned in 911 when the Viking chieftain Hrolf accepted Christianity and became a vassal of King Charles the Simple. In return Hrolf received the lands known today as Normandy, and as Rollo, duke of Normandy, he and his Vikings became settlers and builders instead of invaders and raiders. Ninth-century castles were relatively small and simple affairs designed to safeguard a relatively small number of people and intended as a refuge during times of trouble. A timber tower on its hill or motte, natural or artificial, could serve as a dwelling like the elaborate tower described by Lambert of Ardre (Document 6). The hall and farm buildings stood near the tower. A moat or ditch, earthen ramparts, and stockades surrounded the site that formed the bailey. The owner built the tallest possible tower and the highest walls; he depended on height for observation and defense. Because he expected his enemies to try to enter in the same place he did, he also fortified the gateway to the compound.