Windsor Castle, royal residence in the town of Windsor, in Windsor and Maidenhead unitary authority, about 35 km (22 mi) west of London. It is set on an elevation overlooking the River Thames, and is one of the most popular tourist attractions in the United Kingdom.

A royal residence has stood on the site since Saxon times. In about 1070, around the same time that he built the Tower of London, William the Conqueror constructed a motte and two baileys at Windsor. This was replaced by Round Tower, a massive circular structure, by Henry II. The castle was virtually rebuilt by Edward III in about 1344, as a gathering place for the knights of the newly established Order of the Garter. From then until the 19th century, the castle was considerably rebuilt, altered, and embellished by succeeding monarchs. The castle's appearance today is largely the result of remodelling carried out for George IV in the early 19th century.

The castle as it stands today consists of the Round Tower flanked by two courtyard complexes—the upper ward (to the east) and the lower ward (to the west). The lower ward contains St George's Chapel, begun in 1474 during the reign of Edward IV and completed in 1528 by Henry VIII. The chapel, a splendid example of late Perpendicular architecture, contains the insignia of the Knights of the Garter and is the burial place of Henry VI, Edward IV, Henry VIII, Jane Seymour, Charles I, Edward VII, and George V. The adjoining Albert Memorial Chapel, originally built by Henry III in the 13th century as a memorial to Edward the Confessor, was lavishly restored by Queen Victoria as a monument to Albert, the Prince Consort.

The state apartments in the upper ward include St George's Hall, the Waterloo Chamber, and the private chapel. This part of the castle was badly damaged by fire in 1992. The upper ward also contains the royal library (where an important collection of Old Master drawings are kept). The Throne Room, the Rubens Room, and the Van Dyck Room contain valuable collections of paintings, statuary, and other works of art.

Home Park, about 200 hectares (500 acres) of parkland, stretches out to the south, east, and north of the castle. Frogmore, in Home Park, is a mausoleum dedicated to Queen Victoria and Prince Albert. A tree-lined avenue more than 5 km (3 mi) long leads from Home Park to Windsor Great Park, largely a public park, and Virginia Water, an artificial lake.