A network of concrete huts were used to stretch along the coastline from Portstewart to Magilligan defending the Province from German invasion during World War Two.
Heritage expert James O’Neill remarked that it might seem “silly” now given that the Nazi fleet never came near our shores, but he said the fear at the time was “very real”.
On Saturday he will lead a walking tour which will concentrate on the defence heritage of Castlerock, dating back to the Second World War.
He said: “There were about seven beaches in Northern Ireland in World War Two that were noted as being susceptible to invasion
“The big open shore that invited invasion would have been Portstewart Strand and Castlerock all the way up to Magilligan.
“After Dunkirk a lot of heavy equipment had been lost so they just built lots of concrete pillboxes along the coastline to act as what was called a coastal crust.
“The idea was if the Germans came the landing force could be held up to give the defending units time to gather a counteract force.
“The pillboxes were protected with barbwire and trenches, they were never meant to stop things dead on the shoreline, they were just meant to hold them up for a couple of hours.
“Operation Sealion as it was called by Hitler never came, but the fear was very real locally that this was going to happen.
“At Magilligan they had anti-aircraft landing poles, they weren’t going to all this effort for nothing.
“This was a very real possibility, only in retrospect does it seem a bit silly.”
As well as pillboxes, Castlerock and nearby Downhill were home to two radar sites.
James, a 47-year-old heritage consultant from Belfast, said: “There would have been an unbroken chain of radar sites protecting the UK, detecting incoming bombers or being used for reconnaissance, controlling the skies around Northern Ireland.
“The first one at Castlerock was a general one, then they realised you could fly low and get under the radar so they had to develop Chain Home Low which sits at Downhill, it was more advanced technology.
“It could scan as low as monitoring sea traffic.
“Castlerock radar site was used right into the Cold War until it closed in the mid-50s.”
He added: “Up by Coleraine you have one of the Royal Observer Corps Monitoring Posts.
“There were 1,500 of these small bunkers covering the entire UK for nuclear monitoring purposes.
“It’s amazing the richness and variety of defence heritage around Castlerock and the Binevenagh area.”
In terms of the pillbox structures built during the Second World War, James said: “What we have now is a shadow of what would have been there. You would have had these things every 300 yards or so.
“A lot of them were demolished after the war because they were seen as eyesores.
“There used to be network of them that would have run all the way along the vulnerable beaches.
“Locals are very familiar with them but they disappear from a national view.
“Only since the late 1990s the idea came that we should be protecting those that remain.
“They’re imprints of history living in the landscape.
“Less and less people are still alive who can remember World War Two so the physical remains are becoming more important.”
The Defence Heritage walk in Castlerock has been organised by The Causeway Coast & Glens Heritage Trust – a charity devoted to protecting and enhancing the unique heritage of the area.
The fully booked tour is happening on Saturday afternoon.
The walk of approximately four miles taking in the defence relics of Castlerock will be taking place within Binevenagh Area of Outstanding Natural Beauty.
The event forms part of the annual Castlerock Walkfest.
James commented: “This Castlerock walking tour is just a small aspect of Northern Ireland defence heritage.
“I know how significant it is, but it’s no good just me knowing. We really need to push for conservation, protection and promotion.
“The best way to do that is to have other people feel the same way by engaging with them, talking to them and explaining why these things are important.”
Heritage consultant James said that the size and scope in Northern Ireland’s defence heritage is “remarkable”.
James – also a keen historian who has published several works on the 16th century military history of Ireland – commented: “So much of Northern Ireland’s defence heritage is hidden in plain view.
“There are four airfields up along the north coast - Eglinton, Maydown, Ballykelly and Limavady.
“I think there are 23 all told including the flying boat bases. The level of preservation still to be found is remarkable by national standards.
“Loads of people would go to the market at Nutts Corner but they wouldn’t realise they’re in the middle of this huge airfield.”