The towers pictured were called the Maunsell Forts. They were constructed in the Thames estuary to protect England's coast from German air raids during World War II.
They were operated as army and navy forts, and named after their designer, Guy Maunsell.
They were decommissioned in the late 1950s.
One of the forts is still occupied and managed by the unrecognized Principality of Sealand.