Tower of London
After William conquered England in 1066 AD, he needed a castle to live in when he was in London. He needed a castle that would protect London from invaders, but also a castle that would protect him, William, from angry English people who didn't like being conquered.
To build his new castle, William brought some of the stone over on boats all the way from France, where he had just finished building a castle at Caen, his home town.
William built the White Tower in a Romanesque style, with rounded arches. Long narrow buttresses helped to support the heavy stone walls. Like the walls of Cairo just a little later, William's walls have crenellations on the top - little walls that men could hide behind while they were shooting arrows at their attackers. (Most of the windows you see were added later).
Inside, the White Tower originally was only two stories high. You walked up and down on spiral staircases.
Built into the wall, William had several latrine toilets - just a board with a hole in it, and a long chute down to the ground. William also had a well built into his castle, so he could get water without going outside if the castle was attacked.
Chapel inside the White Tower
One of the rooms was a Romanesque chapel where William could attend Christian Mass without having to leave his safe castle (except that actually William died before his castle was finished).
Tower of London (1200's AD)
In the 1100's, King Richard built another stone wall around the White Tower, to be extra safe, and dug a moat. Later kings also worked on the Tower, and continued to live there until after 1500 AD.