The official name of Tower of London is Her Majesty's Royal Palace and Fortress. According to Wikipedia, it was built in 1078. In different period of time, it was used as a fort, prison, office of archive, palace as well as observatory.
As it is a famous place for torture and execution, the meaning of a phrase "sent to the Tower" is sent to jail. It is good to take a night shot to Tower of London at the south bank of River Thames.