Windsor Castle
In a word, Windsor Castle is spectacular. This 900+ year old castle is just outside London to the west. It is the largest occupied castle in the world. Members of the Royal Family are frequently in residence in the private part of this castle. When the Queen is actually in the castle, her Royal Standard flies above the Round Tower. The remainder of the castle, including many rooms that are in use today are open to the public.
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St. George's Chapel lies completely within the grounds of Windsor Castle. The architecture is spectacular and the wood carvings in the choir are enough reason in themselves to visit the chapel. Quite a few monarchs and members of the Royal Family are entombed here, including Henry VIII, Charles I, Jane Seymour (one of Henry VIII's wives, not the actress), and most recently the Queen Mother.
The changing of the guard ceremony always draws large crowds at the guards and bands march through the city streets and into the castle grounds where the elaborate ceremony takes place.
This is the private area of the castle where the Royal Family live when in residence. The area is well guarded and off limits to the general public. Some of the rooms are actually open to the public, through a separate entrance, but only when not in use for official functions.