William the Conqueror's invasion of England in 1066 set off a wave of Norman expansion that did not stop at the Welsh border. By the early twelfth century, Norman knights were carving out lordships in southern Wales, building fortified manors on whatever land they could hold. The De Costentin family were among them - taking their name from the Cotentin peninsula in northern France, the same flat green land that lies under modern Cherbourg. They built a small fortified manor house on this site - probably not very substantial, since documentary evidence shows it had already fallen into total ruin by 1437. Around the manor grew a cluster of small stone round houses, or crofts, with thatched roofs, holding fifty to a hundred people including children. They called the place Costentinstune. Over the centuries it slurred into Cosmeston.

The villagers had two persistent enemies. The first was water: the land is low-lying, the surrounding springs and streams now feed the modern lake, and even with substantial drainage dykes the medieval villagers struggled to keep the arable fields from waterlogging. Crops failed in wet years. Animals died. Around 1316 the manor passed to another Norman family, the de Caversham, who appear to have brought a more deliberate approach to settlement planning - the excavated 14th-century buildings show signs of design, of a regulated and more compact layout. But the same year, 1316, Llywelyn Bren attacked Caerphilly Castle and triggered two months of warfare across Glamorgan. The de Caversham village had to navigate both flood and politics. Then came the second enemy. The Black Death arrived in Wales in the 1340s. Cosmeston was small and poor, sitting on bad agricultural land, surrounded by water sources that may have spread disease. Like many deserted medieval villages across England and Wales, Cosmeston probably never recovered.

By 1824, when the Marquis of Bute's surveyors mapped the area, all that remained of Cosmeston was four isolated crofts and the Little Cosmeston Farmhouse. Unusually for a Norman settlement, no parish church had ever been established here - which may explain how completely the place was forgotten. Without a surviving church, there was no centre to anchor the local memory. The fields went back to grazing. Over the following century, limestone quarrying chewed away at the surrounding land, and Snocem Concrete works built up around the edges. When both closed in 1970, the council's plan was simply to landscape the scarred ground into a lakeside country park. Then the diggers turned up the foundations, and the archaeologists came.

The excavations of the 1980s established the village's basic shape: a clustered settlement of 14th-century buildings around the older manor site. Working from the foundations and from what was known of medieval Welsh peasant architecture, the project rebuilt the village in situ - thatched stone round-houses on the original footprints, recreating the kind of village that had stood here in the time of the de Caversham family. Today Cosmeston operates as a living history museum, staffed by reenactors who camp in tents around the outskirts and perform displays of historical combat for the public. Visitors walk between the rebuilt crofts, watch wool being spun and bread being baked over open fires, and learn what fourteenth-century Welsh peasant life actually involved: dirt floors, smoky interiors, livestock under the same roof as the family, and the constant low-grade emergency of trying to grow enough food in a flooded valley.

The reconstructed village makes a convincing medieval set, and television production crews have noticed. A 2014 episode of the long-running BBC drama Doctor Who filmed here. So did Merlin, the BBC's fantasy retelling of the Arthurian legends. So did Galavant, the short-lived ABC musical-comedy series that ran for two seasons in 2015-2016 and is fondly remembered by an unreasonably devoted fanbase. The crofts photograph well in winter light or summer haze. The rebuilt thatch reads as authentic on camera. The village's second life as a film location has helped fund its primary function as an open-air museum - run by the Vale of Glamorgan Council, hosting school tours and historical societies, staging special events through the year. The Norman family who built the place would not recognise the visitors. They would probably recognise the buildings.

What you walk through at Cosmeston is an unusual layering of history. The visible buildings are modern reconstructions, but they stand on the original foundations. The pottery and bones found in the excavations were the bones of people who lived and died here seven hundred years ago - peasants who paid rent to Norman lords, worked waterlogged fields, raised children in smoky stone rooms, and were probably buried in unmarked graves nearby when the plague came. The visible reenactors are theatre. The site beneath them is not. The Cosmeston Lakes that the country park is named for fill the worked-out quarries, but they also drown some of the land where the medieval villagers tried to grow grain. Walk to the edge of the modern lake and you are standing where the medieval drainage failed.