William I, known as William the Conqueror (c. 1028-87). King of England. The illegitimate son of Robert I, duke of Normandy, he effectively ruled Normandy from about 1042. On a visit to Edward the Confessor in 1051 he was reputedly promised the throne of England, but on Edward’s death in 1066 Harold succeeded to the throne. William invaded England, killed Harold at the Battle of Hastings, and took the crown. He then followed a policy of consolidation, by building castles and introducing the feudal system. He also organized the countrywide survey known as the Domesday Book.