Windsor Castle, English royal residence that stands on a ridge at the northeastern edge of the district of Windsor and Maidenhead in the county of Berkshire, England. The castle occupies 13 acres (5 hectares) of ground above the south bank of the River Thames. Windsor Castle comprises two quadrilateral-shaped building complexes, or courts, that are separated by the Round Tower. The latter is a massive circular tower that is built on an artificial mound and is visible for many miles over the surrounding flatland. The court west of the Round Tower is called the lower ward; the court to the east is called the upper ward.There was a royal residence at Windsor in Saxon times (c. 9th century). William I ("William the Conqueror") developed the present site, constructing a mound with a stockade about 1070. Henry II replaced this with the stone Round Tower and added outer walls to the north, east, and south. In the 13th century Henry III completed the south wall and the western end of the lower ward and built a royal chapel on the site of the present-day Albert Memorial Chapel. Edward III made this chapel the centre of the newly formed Order of the Garter in 1348 and converted the fortress buildings in the upper ward to residential apartments for the monarchs. These apartments were rebuilt by Charles II and later reconstructed by George IV for use by visitors of state in addition to the monarchs.