Windsor Castle was built in the 11th century by William the Conqueror and is the oldest and largest inhabited castle in the world. It has been used by the British monarchy for almost 1,000 years and is an official residence of Her Majesty The Queen, whose standard flies from the Round Tower when she is at home. Parts are open to the public, including the State Apartments and St George's Chapel. A further highlight is Queen Mary's Dolls' House.
However, because Windsor Castle is a working palace, opening arrangements are subject to change, sometimes at short notice, and you should check before making a special journey.