The London Tower The first structure erected at the Tower of London, the White Tower, was begun by William the Conqueror and finished by his sons in London, England. The White Tower is the Tower of London's main stronghold. It is a large fortress built of white stone with red roofs. The walls are over 18 feet (5.5 m) thick at their base and rise to a height of 70 feet (21 m). They contain three levels of defensive features: an outer wall, an inner dry ditch, and an internal courtyard. The White Tower has been used for many purposes over the years, including prison. Today it is one of the Queen's residences when she is in London. The word "tower" comes from the Old English term tūn, which means "town." Thus, the White Tower is so called because it resembles a fortified town rather than a traditional castle. It was originally built as a palace but has since been converted into a military barracks. There are other buildings inside the Tower but only the White Tower is open to the public. It can be visited without charge on a guided tour that lasts about an hour. There are also self-guided tours available online. You cannot go inside the White Tower but there are some interesting things to see outside its walls.