Thanks to Marc Humby for the picture.

Maunsell's Naval Forts
The sea forts were designed by Mr. Guy Maunsell, a notable architect, who sent numerous design proposals to the Admiralty during the run up to the start of hostilities at the start of the second world war. In October 1940 Maunsell was instructed to design a sea fortress which could be towed into the Thames Estuary and sunk to defend the Southern coast of Britain from any enemy attack via air or sea.
His design used a pontoon base in a single unit 180 foot long by 80 foot wide. This had a weight of 2623 tons. On top of this was located another unit which housed the guns, men and ammunition. This design was estimated to cost around £3,250,000 at 1994 prices for three such units. However the construction of these forts never actually took place.

Shortly after this the threat of a German invasion increased as France fell to the Nazi onslaught. As well as this, Allied shipping in the Channel was becoming targetted more frequenty by the enemy, and German bombers were finding little resistance to their progress as they travelled above the Thames on their way to bomb the London Docks. The admiralty thus asked Maunsell to design blueprints for five new fortresses. These were largely based on his previous work, but was capable of housing 100 men inside the legs of the structure, as well as having a large gun emplacement built on the top. These fortresses were self sufficient for over a month.