Across the British countryside, from riverbanks to hedgerows, cliff-tops to mountainsides, stout concrete buildings still stand guard.
Now largely forgotten and ignored, the humble pillbox suddenly appeared in great numbers across our landscape at the start of the Second World War, as the country prepared for invasion.
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Back in 1940, Britain stood alone. Much of Europe had fallen to the Nazis, and the British Army had been forced to retreat from Dunkirk - invasion seemed imminent. But how do you defend a coastline of more than 11,000 miles with a routed army and very little equipment?
Britain's 1940 anti-invasion defences
The British response was to dig in and, with typical wartime ingenuity, the Army developed a plan that would use the natural landscape to hinder the German advance. Marshes, forests and rivers, alongside canals and railway lines, would all be deployed to stop the Nazis- and pillboxes filled the gaps in between these natural featuresaround the coast, from Scotland down to Cornwall," explains pillbox enthusiast Bob Osborn. "This was the first line of defence, dubbed the Coastal Crust."
These defences used a mixture of First World War gun emplacements, pillboxes and searchlights, while the beaches were covered in barbed wire, mines and even flame traps, which could be lit to set the sea on fire.
What were 'stop lines'
'Stop lines' were the second line of defence, continues Osborn. "Once the Germans had broken through the coast, the idea was to compartmentalise the country.
The stop lines weren't expected to halt a German advance; the idea was that they would slow them down, and that there would be a central reserve of troops that could be rushed anywhere in the country."
More than 50 stop lines were constructed around Britain and we often stumble across the remnants of these when we're out walking. But they were a far cry from the likes of the Maginot Line and other heavily fortified defences in Europe; a lack of time and money meant the British had to come up with a more frugal solution.
"Royal Engineers would have looked at the landscape and then worked out how they could reinforce it with fixed defences," explains Wayne Cocroft, a senior archaeological investigator with English Heritage. "But, of course, these cost time and money, so wherever possible the natural terrain was always used."
The Taunton Stop Line, spanning from Devon in the south to north Somerset, was designed to defend against an invasion from the south-west, explains Osborn. It starts by following the River Parrett, where mudflats would have slowed an advancing army.
With all the bridges mined so they could be destroyed at the flick of a switch (you can often spot bridges where the demolition chambers have been refilled with concrete), the stop line continued until it picked up the Chard Canal, which became the next natural barrier.
Where were pillboxes put?
After Chard, the railway embankment took over until Axminster, where the River Axe completed the line down to the sea. And where there was a weak link, such as around a bridge or railway crossing, a pillbox appeared.
It's for this reason that pillboxes often seem randomly positioned, staring out across open fields. "The Germans weren't necessarily going to come along the road," says Osborn. "The position of some pillboxes may seem ridiculous, but they were invariably put there for a reason... they were usually sited with a fair degree of care, aking into account the lie of the land."
The pillboxes were designed by the Army but largely built by civilian contractors and, in an incredible feat of engineering, around 18,000 sprung up in just six week during the summer of 1940. Many were reinforced with old bed springs, and the most popular design proved to be the Typ 24, a squat, six-sided building with special windows, called embrasures, designed to allow rifle-or machine gunfire.
Conditions would have been cramped for the local Home Guard, which was expected to man them.
Most pillboxes were heavily camouflaged, too. The simplest would be painted, with a camouflage net thrown over the top, while others involved a more intricate makeover - the Army often called on the services of theatrical set designers and artists.
Some were disguised as fishermen's sheds or boating huts; others as tearoom kiosks, while some were even kitted out to look like gypsy caravans.
Many were blended into their natural surroundings, such as the shingle-covered pillbox on the beach at Porlock, Somerset, or those in uplands areas that were dug into the hillside.
Extra defences included massive anti-tank ditches, measuring some 4.5m (15ft) across and 3.6m (12ft) deep, which ran the whole length of the stop lines. Road and rail blocks used huge fixed concrete blocks with large holes, into which railway sleepers could be slotted to create barriers.
Many of the blocks can still be seen lying around the countryside today, as can dragon's teeth, pyramid-shaped blocks designed to hinder tanks
The defences also stretched up into the mountains. As well as the potential for invasion on the south coast, Churchill reasoned that Hitler could invade from the neutral Republic of Ireland, too. As a result, the Welsh valleys were also heavily defended, especially Snowdonia, where a whole clutch of pillboxes remain around the Llanberis Pass.
In some places, says Cocroft, the Army felled trees to act as a barrier, while on Romney Marsh, huge areas were flooded to prevent German troops from landing.
In the end, though, this ingenious network of defences went untested. Operation Sealion was abandoned in June 1941, as Hitler was forced to refocus his attention to the Eastern Front.
What happened to the military defences?
In 1995, the Defence of Britain Project began cataloguing what remained of Britain's 20th-century military defences.
Some 20,000 sites were documented, and since then there have been campaigns to save small pieces of our Second World War history. But, concedes Cocroft: "It's very difficult to get them protected because there's an awful lot of them and people don't associate them with being important."
While many of the coastal defences have slowly slipped into the sea, it's thought that around 6,500 pillboxes still remain. Some have found new uses as cow sheds and even bat roosts, while a handful have been listed, including a whole ring surrounding Tollerton Airfield in Nottinghamshire.
At Waverley Abbey in Surrey, the defences that formed part of the GHQ Stop Line included pillboxes disguised to look like ruins, and in many ways this epitomise the British response to the threat of invasion.
"It's a great juxtaposition of the medieval alongside 1940s Dad's Army defences," says Cocroft. "But they're all part of our heritage, and of our landscape, too."
Main image: Old concrete pillbox on top of rocks on the beach at Cornelian bay with view to Scarborough castle on the distant headland. A feature on the coast of North Yorkshire.
