 Windsor Castle is a royal residence at Windsor in the English county of Berkshire . It is notable for its long association with the English and later British royal family and for its architecture . The original castle was built in the 11th century after the Norman invasion of England by William the Conqueror . Since the time of Henry I , it has been used by all monarchs , and is the longest @-@ occupied palace in Europe . The castle 's lavish early 19th @-@ century State Apartments were described by the 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 " . Inside the castle walls is the 15th @-@ century St George 's Chapel , considered by the historian John Martin Robinson to be " one of the supreme achievements of English Perpendicular Gothic " design . 
