After passing through the main entrance (Byward Tower) and Middle Tower the first section you see actually inside the castle is Bell Tower as shown above. In this picture you can just make out a Beefeater (Yeoman Warder) in the background giving a guided tour of the castle. As you walk around the grounds you will find several guided tours being given with information given on all areas of the castle. You are free to join any of these guided tours.
Bell Tower itself is the oldest of all the Towers and thought to have been constructed by William Longchamp in the 13th century. It gets its name from the Belfry at the top of the tower (originally it was used as an alarm). These days it rings each evening at closing time for the public. Bell Tower was used as a prison and its inmates included Sir Thomas More..