Windsor Castle is the current seat of the Royal family, being the main residence in which the King resides in taking care of State matters, aside from St. James' Palace and Buckingham House.
The original version of the castle was built in the 11th century during the reign of William the Conqueror. Since Henry I, it has been used a Royal residence for the reigning English, and eventual British Royal Families. The castle was initially designed for the protection of Norman dominance around the outskirts of London, and to oversee a particularly strategic part of the River Thames.
Windsor Castle holds history unique to itself. The structure survived the 1st Barons War, at the start of the 13th century. It would, later, during the English Civil War, be used by the Parliamentarians as a military headquarters, and a prison for King Charles I.
At the restoration of the Monarchy, King Charles II rebuilt much of Windsor Castle at a fairly large expense, upgrading it to a much more Baroque style.
After a period of neglect, upon George III's ascension to the throne, he would do like Charles II, renovating Charles' palace at a collosal expense, adding Rococo, Baroque, and Gothic architecture, which would be the design of the State Apartments, located opposite of the King's Gateway.