With its collection of red-brick houses on neat streets, it seems a perfectly ordinary housing estate. But builders excavating for new homes on the edge of Monmouth in Wales have unearthed a staggering archaeological discovery.

They have found a fortified farmhouse built 4,900 years ago on a manmade island in an Ice Age lake.

It’s thought the fort – known as a ‘crannog’ – was raised on stilts to protect it from attack by neighbouring tribes.

Radiocarbon dating of the timbers reveal the fortified farmstead at Monmouth, South Wales, was built in 2917 BC - making it 300 years older than the Pyramids at Giza. The wooden fort was built on stilts standing over a long-vanished Ice Age lake 4,800 years ago to defend against marauding tribes (artist's impression pictured)

The lake has long disappeared, but the fort’s timbers survive. And, using radiocarbon dating, scientists have determined the house was built in 2917 BC.

That’s 300 years before the Pyramids were built in Egypt and the stones of Stonehenge were raised. Crannogs – natural or artificial islands, enclosed by a ring of stakes – have been found in Scotland and Ireland. But most of the 600 in Scotland are from much later.

Until now, there has been only one crannog found in England or Wales: not far from Monmouth, at Llangorse Lake in the Brecon Beacons. But that one – now reconstructed – is thousands of years younger.

This find – made during excavations for a Barratt Homes development – comes two years after a neighbouring prehistoric boat-building site, thought to be among the oldest in the world, was found.

The wooden fort was built on stilts standing over a long-vanished Ice Age lake 4,800 years ago to defend against marauding tribes. Pictured, example of a crannog

In 2013, three 100ft channels, the width of a canoe, were found dug into the earth. Dating from 1700 BC, they are thought to be grooves in which a twin-hulled boat was repaired, with its supporting beam in the third channel.

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As the archaeological puzzle clicks into place, you see that these Stone Age men must have depended on boats, with crannogs stranded in the lake that once existed where the outskirts of Monmouth now lie.

When the boatyard is taken with the crannog, this corner of south-east Wales begins to look like a cradle of British civilisation. ‘This is surely one of the most stunning of prehistoric discoveries,’ says Stephen Clarke, chairman of Monmouth Archaeological Society and author of The Lost Lake – Evidence of Prehistoric Boat-Building.

‘An exceptional feature is that the construction was based on three massive parallel “sleeper beams” – timbers roughly hewn from complete trees set in the ground horizontally. One of these timbers is a metre wide and all of them seem to have been from full-grown trees.’

Another clue to the crannog’s age is the tool used to carve the oak – a stone axe. Before the Bronze Age stone and flint were used for cutting. You can still see the marks left by stone tools in one of the preserved timbers of the crannog. It was a skilful piece of work – the oak post, or stilt, has been meticulously carved to form a flat surface which would probably have been laid on top of a wooden foundation in the bed of the lake.

Crannogs were in prosperous developments, reserved for the grandest of the grand – the crannog at Llangorse Lake might have belonged to a Dark Ages king.

Crops and cattle would have been in fields, with sheep on the pastures in the surrounding hills. Neighbouring woods would have provided wild cabbage, fruit, wild boar and hazelnuts. And then, if under attack, the tribe could retreat to the safety of the crannog with its vast, natural moat of the lake.

The discovery of the crannog under a building site in Monmouth, South Wales (pictured) paints a picture of a thriving community based around the world's oldest known boat building yards. The prehistoric boat building site was uncovered in 2013 near the newly-discovered crannog site - and dates back to 1,700 BC

Remains of the crannog timbers (pictured) were preserved beneath the clay and peat of a lagoon which formed when the lake drained some 2,000 years ago. They had been skilfully worked with a stone axe to form the bottom of a stilt supporting structure in the lake bed

Today Monmouth is on dry land, at the meeting point of the rivers Wye, Trothy and Monnow. But when the crannog was built, much of the town would have been 20ft underwater, with pebbly beaches running along what is now Monnow Street and St James Street.

For centuries after the crannog was built, it attracted Stone Age, Bronze Age and Iron Age tribes. A Stone Age hearth from about 2795 BC has been discovered in Monmouth, along with animal bones and charcoal from around 500 BC and Roman pottery.

Stone Age artefacts are thin on the ground in Britain. So the discovery of the Monmouth crannog will shed a vast amount of light on the last stage of the Stone Age. This was when hunter-gatherers began to settle in recognised settlements. Animals and plants were domesticated – and Britain assumed a rough approximation of its modern look. The great forest that covered much of the country was gradually cleared, as grassland was created for grazing and growing cereals.