Hoping to thwart any attack launched against them, the builders of European castles incorporated as many security features as they could into their basic design, including ways to stop besiegers from getting over, under, or through the walls and other defenses. Some of these features became outdated when new tools for waging of ensive sieges were invented. h e defenders then had to come up with ways to neutralize those new tools. In turn, this stimulated the attackers to try to i nd still more ways to outsmart the besieged, and so forth. Siege warfare in medieval Europe therefore contained a kind of military arms race related to penetrating fortii cations on the one hand and to maintaining their security on the other. Few castles existed in Europe before the Battle of Hastings and the Norman conquest of England. Afterward, most of the early fortresses were motte and baileys, consisting mainly of small earthen mounds and wooden stockades. Duke William’s initial intention in erecting these structures was to use them as guard posts to watch over and control specii c regions. A clear example of the ef ectiveness of this approach was William’s i rst large motte and bailey, established at Berkhamsted, 25 miles (40 km) northwest of London, in the late fall of 1066. Erected in only a few weeks, the castle featured a motte about 40 feet (12 m) high. h e bailey was about 500 by 300 feet (150 by 91 m) in extent and surrounded by a tall stockade. At least one water-i lled moat ran along the perimeter of the fence. h is imposing structure, garrisoned by several hundred soldiers, was part of William’s i rst major goal following the i ght at Hastings. He hoped to persuade the Saxon nobles who then controlled London, and who still opposed him, to surrender the city. Placing the castle beside the key road leading from London northward into the Midlands (south-central England) was meant to send a message to them. William knew that the native Saxons had no large-scale military installations of their own and correctly reasoned that the sudden appearance of such structures would intimidate the locals. As British historian Geof rey Hindley puts it, the i rst Norman castles sent “an unmistakable signal to a conquered people to heed the alien oppressor and robber of their liberty.”24 h us, seeing the new motte and bailey at Berkhamsted, along with other castles rising nearby, the nobles in London realized they were outmatched. h ey soon surrendered to William without a i ght. It appears that England’s new Norman ruler had planned all along to eventually replace most of his motte and baileys with stronger, more permanent stone versions. h is advanced castle-building program began in the last decades of the eleventh century and inl uenced builders across Europe. Archer Jones describes the i rst Norman stone fortresses, which were fairly small and simple compared to later versions: “h e early Norman castle,” he writes, “concentrated almost all of its strength in a single enormous tower known as a donjon or keep.” h is impressive structure “combined height and a maximum space inside with only a narrow perimeter to defend. Defenders needed only men enough to man the battlements and to drop things down on anyone trying to sap [dig beneath] the base of the wall.”25 Most of the Norman stone keeps were square in shape. Each had a small square turret—a defensive box or projection—rising upward from the top of each corner. h e keep acted either as a secure residence for the owner or a last refuge for the owner and other upper-class residents of the area during an emergency. Most often a narrow courtyard surrounded the keep. h at open space itself was lined by tall defensive walls, often termed curtain walls. Inside these barriers the builders installed workshops, stables, storerooms, and living quarters for the soldiers who manned the defenses. Meanwhile, the outer walls’ perimeter was frequently lined with a deep, water-i lled moat intended to keep attackers away from the walls. Extra courtyards and sets of defensive walls were added to many of the i rst stone fortresses in the two centuries that followed, even while much larger and more complex castles were built from scratch.