It is one of the main official residences of the British royal family, located in the Royal Borough of Windsor Maidenhead in Berkshire, southeastern England. This historic castle was first built in 1070 by William I to protect the safety of ships and the royal family on the Thames River. After expansion and renovation by successive monarchs, Windsor Castle has become one of the largest inhabited castles in the world. Not only is it the administrative residence of the British monarch, but also the venue for their official and private activities. Queen Elizabeth II often spends weekends in the castle, which is also frequently used to receive foreign heads of state. He is a witness to British history.