Windsor Castle, Windsor, Berkshire, England. Windsor Castle is the oldest and largest occupied castle in the world. It is the favorite weekend residence of Queen Elizabeth II. Windsor Castle was built in the 11th century by William the Conqueror, and has been occupied by English or British royals since the time of Henry I. It occupies 13 acres, and acts as a fortress, Royal residence, and town. It has survived multiple wars, a major fire, and multiple restorations. Although it is a working Royal Palace, it is open to tourists for much of the year.