Windsor Castle in Windsor and Maidenhead

You're standing at Windsor Castle, the largest inhabited castle in the world. In the middle stands the Round Tower, sitting on a man-made mound fifteen metres high. The mound was built from chalk dug out of the ditch around it. The tower isn't actually round, because of the shape of the mound underneath. If the Royal Standard is flying from it, the monarch is at home. If you see the Union Jack, they're not.