The Tower of London was a London landmark which was built as a castle in the early centuries.
Henry VIII once sent the Doctor to the Tower of London, which is where the Doctor wanted to be as that's where his TARDIS was. (DW: The Sensorites)
The Doctor was later imprisoned in the Tower of London again, where he shared a cell with Sir Walter Raleigh. (DW: The Mind of Evil)
In 2006, the secret base of the United Nations Intelligence Taskforce was located beneath the Tower of London. (DW: The Christmas Invasion)