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29th September 1945: Three of the seven linked forts in the Thames Estuary which guard British shipping lanes against mines and mine laying aeroplanes. Primarily erected as first line invasion defences, each tower is armed with a 3.5 artillery gun. The forts were constructed ashore, towed out to sea, sunk on sandbanks and then joined together with cat walks. (Photo by Fred Morley/Fox Photos/Getty Images)