At the outbreak of World War II, the Port of London was the busiest port in the world. The German Navy recognized the Thames as an important shipping route and sought to disrupt its usage through the use of a new secret weapon — the magnetic influence mine. Essentially these mines were detonated by the presence of a large metallic object, like that of a steel-hulled ship. This meant that the mines could detonate with ships in close proximity, not having to actually make physical contact to be effective.
Hundreds of ships were sunk by these innovative mines and the Allies needed a solution, fast. Knowing the mines were laid by aircraft, Guy Maunsell (1884-1961), a British civil engineer, produced plans for offshore defences. After some modifications his plans were approved and the Maunsell Sea Forts and Army Towers were born. [Source: Project Redsand]