The castle remains centered on the motte or artificial hill on which William the Conqueror built the first castle. Henry II constructed the Round Tower and the original stone outer wall, ca. 1165.

One of a series of castles that William I (reigned 1066-1087) established around London, Windsor occupied the nearest strong point in the Thames Valley to the west of the city. By the reign of Henry I (1100-1135) the creation of a large hunting forest, together with the proximity of London, made this a favoured royal residence as well as a fortress. It is the largest inhabited castle in the world and the oldest in continuous occupation. In 1992 a fire damaged approximately one-fifth of the castle; restoration was completed in 1997.