WALKING at the Naze you may notice the n-shaped object out at sea to the north east.

It is a relic of the Second World War, one of four naval sea forts designed by Guy Maunsell for the British Royal Navy to defend key shipping lanes from German attack.

It was the first of originally four naval forts designed to protect the Thames Estuary. The sea fort was constructed in dry dock at Red Lion Wharf, Gravesend, in 1941. It was originally HM Fort Roughs or Roughs Tower during the war, with between 100 and 120 naval officers stationed there.

In February 1942 the fort was towed to a location on Rough Sands – a patch of shallow water approximately ten miles off Harwich.

Its base was then intentionally flooded so that it sank in about 11 metres of water, coming to rest on the sandbar.

This artificial naval installation is similar in some respects to early fixed offshore oil platforms. The twin concrete supporting towers were divided into seven floors, four for crew quarters; the remainder provided dining, operational, and storage areas for generators, fresh water tanks and anti-aircraft munitions.

The towers were joined by a steel platform deck on which anti-aircraft guns were mounted.

At the conclusion of war-time hostilities all original military personnel were evacuated from HM Fort Roughs. It remained deserted until 1966 when Roy Bates, who operated Radio Essex, and Ronan O'Rahilly, who ran pirate station Radio Caroline, occupied the site and declared it an independent mini state.

The government sent in Royal Marines and ordered Bates to surrender and leave what they claimed was Crown property. He and his son were arrested and charged, but the courts threw out the case as they did not have jurisdiction over international affairs because Roughs Tower lay beyond the legal territorial waters of Britain.

Bates took this as de facto recognition of his country and later issued a constitution, flag, postage stamp and national anthem.

Fifty years ago this month, in 1967, the Principality of Sealand was founded and is still occupied. The current owner is Roy's son Michael. The old rusting outpost has operated as offshore storage for electronic records, and sells aristocratic titles and Sealand paraphernalia.

Among its many title-holders is the singer-songwriter Ed Sheeran, who is Baron of Sealand.