Windsor Castle is a medieval castle and royal residence in Windsor in the English county of Berkshire, notable for its long association with the British royal family and for its architecture. The original castle was built after the Norman invasion by William the Conqueror. Since the time of Henry I it has been used by a succession of monarchs and 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, making it the largest inhabited castle in the world.

Originally designed to protect Norman dominance around the outskirts of London, and to oversee a strategically important part of the River Thames, Windsor Castle was built as a motte and bailey, with three wards surrounding a central mound. Gradually replaced with stone fortifications, the castle withstood a prolonged siege during the First Barons' War at the start of the 13th century. Henry III built a luxurious royal palace within the castle during the middle of the century, and Edward III went further, rebuilding the palace to produce an even grander set of buildings in what would become "the most expensive secular building project of the entire Middle Ages in England". Edward's core design lasted through the Tudor period, during which Henry VIII and Elizabeth I made increasing use of the castle as a royal court and centre for diplomatic entertainment.