These strange contraptions are armored towers built in World War II. They are located just offshore from the Thames and Mersey River estuaries, designed to report air raids or invasions. There were seven fort locations off the east coast of England. Since being de-commissioned in the 1950s, one of the towers has been used for pirate radio broadcasts and another has been claimed by the Principality of Sealand, an unrecognized micronation.
https://en.wikipedia.org/wiki/Maunsell_Forts
Photo: Creative Commons (Wikimedia), uploaded by Russss