Windsor Castle is a royal residence at Windsor in the English county of Berkshire. The castle is notable for its long association with the British royal family and for its architecture. The original castle was built in the 11th century after the Norman invasion by William the Conqueror. Since the time of Henry I, it has been used by succeeding monarchs and it is the longest-occupied palace in Europe. The castle's lavish, early 19th-century State Apartments are architecturally significant, described by art historian Hugh Roberts as "a superb and unrivalled sequence of rooms widely regarded as the finest and most complete expression of later Georgian taste". The castle includes the 15th-century St George's Chapel, considered by historian John Robinson to be "one of the supreme achievements of English Perpendicular Gothic" design. More than five hundred people live and work in Windsor Castle.