The essence of Norman feudalism was that the land remained under the lord, whatever the vassal might do. The lord had the duty to defend the vassals on his land. The vassal owed military service to the lord and also the service of attending the courts of the hundred and the county [formerly "shire"], which were courts of the King, administering old customary law. They were the King's courts on the principle that a crime anywhere was a breach of the King's peace. The King's peace that had covered his residence and household had extended to places where he might travel, such as highways, rivers, bridges, churches, monasteries, markets, and towns, and then encompassed every place, replacing the general public peace. Infraction of the King's peace incurred fines to the King. This feudal bond based on occupancy of land rather than on personal ties was uniform throughout the realm. No longer could a man choose his lord and transfer his land with him to a new lord. He held his land at the will of his lord, to be terminated anytime the lord decided to do so. A tenant could not alienate his land without permission of his lord. In later eras, tenancies would be held for the life of the tenant, and even later, for his life and those of his heirs. This uniformity of land organization plus the new requirement that every freeman take an oath of loyalty directly to the king to assist him in preserving his lands and honor and defending him against his enemies, which oath would supersede any oath to any other man, gave the nation a new unity. The king could call men directly to the fyrd, summon them to his court, and tax them without intervention of their lords. And the people learned to look to the king for protection from abuse by their lords. English villani, bordarii, cottarii, and servi on the land of the barons were subjugated into a condition of "villeinage" servitude and became "tied to the land" so that they could not leave the land without their lord's permission, except to go on a pilgrimage. The villeins formed a new bottom class as the population's percentage of slaves declined dramatically. They held their land of their lord, the baron. To guard against uprisings of the conquered people, the barons used villein labor to build about a hundred great stone castles, with moats and walls with towers around them, at easily defensible positions such as hilltops all over the nation.