Windsor Castle  was originally built by William the Conqueror in the 11th century, and it has since been the home of 39 monarchs. It is open to visitors throughout the year. The castle is 5.3 hectares/13 acres in size and is a breathtaking sight indeed. It is the largest and oldest occupied castle in the world, and it’s where Her Majesty the Queen chooses to spend most of her private weekends.