The Maunsell forts were fortified structures designed by Guy Maunsell and built in the Thames and Mersey estuaries during World War II. They served as anti-aircraft defenses, primarily for defending against German Luftwaffe attacks on London. These forts were constructed of steel and concrete and were designed to be floated out to sea and anchored. After the war, some were repurposed as pirate radio stations. The forts came in two main designs: naval forts and army forts. Naval forts were large, single-platform structures, while army forts were groups of interconnected steel towers.