I’d never heard of the Maunsell Sea Forts until tonight. When I first saw this picture, I thought it was an image from Myst or Riven:
The Thames Estuary Army Forts were constructed in 1942 to a design by Guy Maunsell, following the successful construction and deployment of the Naval Sea Forts. Their purpose was to provide anti-aircraft fire within the Thames Estuary area. Each fort consisted of a group of seven towers with a walkway connecting them all to the central control tower. The fort, when viewed as a whole, comprised one Bofors tower, a control tower, four gun towers and a searchlight tower.
And, of course, there’s an article on it at Wikipedia.