The Tower of London is a landmark in central London, in the London Borough of Tower Hamlets, just outside the City of London. The White Tower, the square building with turrets on each corner that gave it its name, is actually in the middle of a complex of several buildings along the River Thames in London, which have served as fortress, armoury, treasury, zoo/menagerie, mint, palace, place of execution, public records office, observatory, refuge, and prison, particularly for upper class prisoners. This last use has led to the phrase "sent to the Tower" meaning "imprisoned". Elizabeth I was imprisoned for a time in the Tower during her sister Mary's reign; the last use of the Tower as a prison was in 1952, serving to restrict the movement of the Kray Twins who had deserted from the army.