The Jewel Tower is the sole remaining structure of the medieval Palace of Westminster, still easily accessible to the public on most days. Built for King Edward III in 1366, it was originally designed as the king’s private treasury before becoming a store for parliamentary records in 1600. A channel brought the river to it’s base (left main picture) providing moated security and fresh fish! It may not be as extensive as the Tower of London, but being only 100m from Big Ben it’s too good to miss!
For more information see: https://www.english-heritage.org.uk/visit/places/jewel-tower/
Photos (c) Essential London