Maunsell sea forts

BLDGBLOG has a post on these World War II forts in the Thames Estuary.

It quotes the Underground Kent site on their history:

Their purpose was to provide anti-aircraft fire within the Thames Estuary area. Each fort consisted of a group of seven towers with a walkway connecting them all to the central control tower.

The fort, when viewed as a whole, comprised one Bofors tower, a control tower, four gun towers and a searchlight tower. They were arranged in a very specific way, with the control tower at the centre, the Bofors and gun towers arranged in a semi-circular fashion around it and the searchlight tower positioned further away, but still linked directly to the control tower via a walkway.

All the forts followed this plan and, in order of grounding, were called the Nore Army Fort, the Red Sands Army Fort and finally the Shivering Sands Army Fort.

All three forts were in place by late 1943, but Nore is no longer standing. Construction of the towers was relatively quick, and they were easily floated out to sea and grounded in water no more than 30m (100ft) deep.