Red Sands Forts
Maunsell army forts were designed and built for anti-aircraft defence. They comprised seven interconnected steel platforms. Four towers arranged in a semicircle ahead of the control centre and accommodation each carried a QF 3.75 inch gun, a tower to the rear of the control centre had Bofors 40 mm guns, while the seventh tower, set to one side of the gun towers and further out, was the searchlight tower. Red Sands Fort (U6) is at the mouth of the Thames Estuary. The towers had been connected by metal grate walk-ways. In 1959 consideration was given to refloating the Red Sands Fort and bringing the towers ashore but the costs were prohibitive. In the early 21st century, in response to proposals to demolish the fort, a group called Project Redsands was formed to try to preserve it. It is the only fort that can be visited safely from a platform in between the legs of one of the towers. https://en.wikipedia.org/wiki/Maunsell_Forts#Red_Sands_Fort_(U6) They all have solid tops and all