The round building on top of the man-made hill is the beginning of the castle, and all the space that William the Conqueror needed to call home in the 11th century. Subsequent monarchs needed a bit more elbow room, so the castle grew in all directions and now resembles a walled city. Of course the castle is also afflicted with a changing of the guards ceremony, although this is much more low key than its Buckingham counterpart.