Windsor Castle is the royal residence at Windsor, in the English county of Berkshire. This castles is known as one of tree residences of the British Royal Family. It is the biggest populated castle known. The castle is 45.000m2 big. This castle was built by William the Conqueror in the 11th century after the Norman invasion. It is build on a man made tump, which is of a strategic importance for controling the river Thames and the main road to London. At first it was surrounder by a palisade and a moat, later stone walls were raised during the reign of Henry I. During the time of his rule this castle became the residence of the British Royal Family, which makes it the longest populated castle to date.