In western Europe in the ninth century, the breakup of the Carolingian empire made the need for personal protection and fortified homes necessary again. The amount and intensity of local warfare increased, and later the attacks of the Vikings from the north and the Magyars from the east made the defense of borders mandatory. In the ninth century the Norse Vikings, masters of the hit-and-run offense, dedicated their skills to ship building and erected relatively light fortifications. They built circular base camps and trading centers that became true cities. When in the tenth and eleventh centuries these Northmen settled in northern France and began to fortify their holdings, they continued to build in earth and timber. They developed the motte and bailey castle, a form popular with the Germanic peoples all over Europe but most often associated today with the Normans.