The Tower of London was a London landmark.
History
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 had also been taken. (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)
The Doctor would end up there yet again, this time with Nyssa, after running afoul of Sir Isaac Newton. (BF: Circular Time)
In 2006, a secret UNIT base was located beneath the Tower of London. (DW: The Christmas Invasion)
