The Tower of London was a London landmark.
The Doctor and the Tower
Prisoner
Henry VIII once sent the First Doctor to the Tower of London, which is where the Doctor wanted to be, as his TARDIS had also been taken there. (DW: The Sensorites) This was not, however, the Doctor's only time as a prisoner in the Tower. On a separate occasion, he shared a cell with Sir Walter Raleigh. Though the event was recounted by the Third Doctor to Jo Grant, it was unclear which version of the Doctor had spent time with Raleigh. (DW: The Mind of Evil) Later still, the Fifth Doctor would return to a cell there — this time with Nyssa —after falling foul of Sir Isaac Newton. (BFA: Summer)
Visitor
The First Doctor assisted Lady Jane Grey and was present at her execution at the Tower (VD: The Nine-Day Queen), but she was also befriended by new lady-in-waiting Rani Chandra (SJA: Lost in Time). While exiled to Earth, the Third Doctor once helped the Brigadier track down an alien master of disguise who had a secret base at the Tower Hill Underground station. From this base, directly under the Tower, the thief hoped to gain access to the Crown Jewels. Although a combination of the Doctor, UNIT soldiers and Beefeaters were unable to capture the thief, they did prevent him from stealing the jewels. (PHS: Secret of the Tower)
Additional history of the Tower
By 2006, UNIT headquarters was located beneath the Tower of London. (DW: The Christmas Invasion) The headquarters was still used in 2009, though UNIT also maintained mobile bases in lorry trucks. (DW: The Sontaran Stratagem)
When The Department controlled Britain around the year 2050, the Tower of London became Dauntless Prison, a prison for all the aliens that arrived on Earth. When K9 Mark 2 and his friends released the alien prisoners there in 2050, the Department's treatment of the prisoners was leaked to the media and the prison was shut down. (K9TV: Liberation)
Sometime around the 29th century onwards, the tower was incorporated into Starship UK, its dungeon containing the devices used to torture the Star Whale beneath the craft into moving. (DW: The Beast Below)
