From the Steam Geek archives-
The Maunsell Towers were set up to provide anti aircraft defence in the Thames Estuary during World War Two. Each “fort” comprised of one Bofors tower, a control tower, four gun towers and a searchlight tower. Three forts were built Nore Army Fort, the Red Sands Army Fort and the Shivering Sands Army Fort, of which the latter two still stand, more or less.
Sealand, most famous of the offshore forts, with plans