William built castles to protect his barons from attacks from unhappy Englishmen. The first castles were called motte and bailey castles. Each castle took 7-14 days to build and by 1086, William's barons had built over 100 castles!

William the Conqueror and his Norman barons built castles primarily as a tool for military subjugation and psychological dominance following the invasion of 1066. As an occupying force in a hostile land, the Normans were vastly outnumbered by the Anglo-Saxon population. The initial "motte-and-bailey" castles—made of earth and timber—could be erected in just a few weeks, providing immediate security for Norman garrisons against local revolts. These structures served as "islands of power" from which the barons could administer their new estates and collect taxes. Beyond defense, the castles were symbols of the new social order; their towering presence on the landscape served as a constant reminder of the king's authority and the permanence of the Norman Conquest. As William consolidated his rule, he ordered these wooden forts to be rebuilt in stone, such as the iconic White Tower at the Tower of London, to create indestructible administrative hubs. This massive building program transformed England's landscape, effectively ending the era of decentralized Saxon rule and establishing the feudal system that would define the Middle Ages.