An area within which the Lieutenant of the Tower of London could muster a militia

The name Tower Hamlets sounds as if it is a modern construct, chosen to bind together a number of former councils into a new one in 1965. In fact the name had been used since 1605 to describe an area within which the Lieutenant of the Tower of London could muster a militia. It continued to be used for administration of justice, and was the name given to the 1832 parliamentary borough. 

T. Travers, London's Boroughs at 50 (2015), 125