Where and what are the Maunsell Sea Forts?
These structures originate from the days of WWII. They were built like oil platforms in the estuaries of the Thames and Mersey Rivers to provide protection from foreign, especially German, ships entering English ports. They were built on concrete towers. The structures had floors for naval crew quarters, dining areas, operations, and storage. They housed generators, fresh water tanks, and antiaircraft guns. Each had a landing area and a crane to hoist supplies on board.
The Maunsell Army Forts are 7 buildings mounted in the middle of the estuaries. When it was up and running and functional all the buildings were connected by a catwalk. It's an engineering marvel designed by Guy Maunsel who was later known for designing and building concrete bridges. There are only two of these structures standing now - the Redsands Fort and the Shivering Sands Fort.
After WWII ended, the forts were decommissioned in 1950. A decade later these forts were famously used as Pirate Radio stations. These were basically Army stations and now they are abandoned.
In 2003, the forts were looked at for preservation and restoration. The Redsands Fort was in a much better state than the Shivering Sands Fort. In 2005, a popular artist named Stephen Turner stayed there for 36 days which was the same length of time that Army servicemen would have stayed at the Fort. He later wrote a book about his whole experience.
It's not illegal but it is extremely hazardous to try to enter one. Even though it is miles away from the coast you can still view them on a good day from the shore.