Windsor Castle is a royal residence at Windsor in the English county of Berkshire. It is strongly associated with the English and succeeding British royal family, and embodies almost a millennium of architectural history.

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 (who reigned 1100–1135), it has been used by the reigning monarch and is the longest-occupied palace in Europe. The castle's lavish early 19th-century state apartments were described by early 20th century 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.

Originally designed to project Norman dominance around the outskirts of London and 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 make 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.

Windsor Castle survived the tumultuous period of the English Civil War, when it was used as a military headquarters by Parliamentary forces and a prison for Charles I. At the Restoration of the monarchy in 1660, Charles II rebuilt much of Windsor Castle with the help of the architect Hugh May, creating a set of extravagant Baroque interiors. After a period of neglect during the 18th century, George III and George IV renovated and rebuilt Charles II's palace at colossal expense, producing the current design of the state apartments, full of Rococo, Gothic and Baroque furnishings. Queen Victoria made a few minor changes to the castle, which became the centre for royal entertainment for much of her reign. It was used as a refuge by the royal family during the Luftwaffe bombing campaigns of the Second World War and survived a fire in 1992. It is a popular tourist attraction, a venue for hosting state visits, and was the main residence of Elizabeth II from 2011 to 2022.