As I recall, this was one of a series of forts built during WWII for air defence. After the war ended, they were abandoned.
The attempt to declare it an independent state foundered when no-one was willing to recognise it as such. I think they tried to raise money by selling "passports", and postage stamps and the like.
From time to time, Martello towers (built for coastal defence during the Napoleonic Wars) come up for sale. Solidly built, they still make good holiday homes. And recently, I think I've seen an advertisment for an off-shore defence fort somewhere off the south coast of England which has had a heli-pad fitted. Just the thing for a reclusive Steampunk who needs a Zeppelin mooring....
