source : http://www.royal.gov.uk/
Windsor Castle is the oldest and largest occupied castle in the world.
Over a period of nearly 1,000 years it has been inhabited continuously, and altered and refurbished by successive monarchs. Some were great builders, strengthening the Castle against uprising and rebellion; others, living in more peaceful times, created a palatial Royal residence.
William the Conqueror chose the site, high above the river Thames and on the edge of a Saxon hunting ground. It was a day's march from the Tower of London and intended to guard the western approaches to the capital.
The outer walls of today's structure are in the same position as those of the original castle built by William the Conqueror in the 1070s. So too is the central mound supporting the Round Tower and the Upper Ward, where successive monarchs have had their private apartments since the fourteenth century.
In the 1170s Henry II rebuilt - in stone instead of wood - the Round Tower, the outer walls of the Upper and most of the Lower Ward, and the Royal apartments in the Upper Ward.
In the 1360s Edward III, who was born at Windsor, extended the Castle. He created the immense St. George's Hall for the use of the Knights of his newly founded Order of the Garter.