The Chateau de Guedelon is not a movie-set; it is not a restoration; it is not a theme park. Guedelon will be a full-sized castle with battlements and a moat and drawbridge and six towers It will be constructed from scratch, by strictly medieval methods, over the next 25 years - the first castle of its kind to be built since the 13th century and the first medieval castle built anywhere for 500 years.
The 35 labourers - many of them formerly unemployed - work in medieval clothes. They eschew modern building methods (modern in this case being anything invented after circa AD1240). All materials, and many of the tools, are quarried, gathered or made on the site: the stones, the mortar, the ropes, the nails, the saws, the timber scaffolding, the wooden lifting- engines.
After 18 months, the 10ft thick foundations and lower walls have been completed. In the next few weeks, the first tower will rise. When the castle is finished, in 2025, its creators hope to add a medieval village and then a priory.
A pall of yellowish golden dust hangs over the site, adding, cinematically but unintentionally, a sepia-tinge. If you squint your eyes to blot out the other visitors, in cycling shorts and baseball caps, and focus instead on the castle walls, the rough shacks and clothes of the labourers, the timber scaffolding, the wood-smoke from the forge, the errant goats and geese, you might have stepped back eight centuries.
The Chateau de Guedelon is the realisation of the dream of a self-confessed "lover of old stones", Michel Guyot, a local entrepreneur, who has restored the nearby Chateau de Saint-Fargeau and is trying to rescue 15 other romantic but unwanted "ruins".
France already has an embarrassing surfeit of castles. Visits to the lesser known chateaux of the Loire are falling dramatically. Why go to the bother of constructing another one?
"Anyone who goes to a medieval castle or cathedral cannot help but wonder how on earth our ancestors built such things," said Mr Guyot, 51, a playful, boyish but determined man. "For many years, I wanted to find out by doing what they did, by building a medieval castle in the medieval way. One day, I made up my mind that it must be now or never. You only have one life."
His aim is to solve some of the mysteries about how such castles were built; to recreate old skills; to provide jobs and a tourist attraction. The chosen era is the first third of the 12th century, a period of intensive castle-building by upwardly mobile barons in France (and England). A committee of academic experts advises on what is medievally correct. Initial funding came not from pillaging the local peasantry but from regional councils, the European Union and several large companies. Guedelon, 100 miles south- east of Paris, has to fund itself from entrance fees, by the end of next year.
The castle is not just an architectural experiment but also a "social experiment", according to the site manager, Maryline Martin. How do you adapt the 35-hour working week, laid down by French law, to a medieval building site? How do you cope with modern regulations on safety in the workplace? What exactly is medievally correct? Which modern habits and practices are acceptable or at least unavoidable? Can the workers have cigarettes? (yes). Watches? (no). Spectacles? (yes). Hard-hats? (sometimes). The 35-hour week is respected, by calculating an annual average. Work closes down in the winter, when the labourers and craftsmen undergo training (including training in modern building techniques) and enjoy something undreamt of by their 13th-century predecessors: paid holidays.
One issue still unresolved is the use of the two wooden cranes, built according to surviving medieval plans, which are operated by human tread- mills, known as "squirrel cages". The work inspectorate of the 20th-century French state declared them unsafe and inhuman. Negotiations are in progress with the "works delegates" (another, acceptable post 13th-century intrusion) to allow the builders to take turns operating them for 15 minutes at a time.
"What has been fascinating is to see the modern craftsmen and labourers adapt to a medieval concept of time," Ms Martin said. "There is no rush; no pressure. And yet if anything we are ahead of schedule. No one sneaks off early or spends longer than they are supposed to at lunch. There are no quarrels or screaming matches. The builders have themselves rediscovered the importance of team-work. If one craftsman is missing, say the blacksmith, and we run out of nails, the whole site has virtually to close down."
The number of work accidents is officially recorded as 50 per cent less than on modern building sites. There have been no strikes.
By trial and error, the late 20th-century builders have solved two mysteries which have long defeated, or divided, architectural historians. What kind of mortar was used in medieval buildings? What are the strange, cabalistic inscriptions on the dressed stones?
It turned out, after several false starts, that the kind of mortar which works best is a mixture of quicklime and rough sand. (It also turns out that its chemical properties match almost exactly those of old pieces of mortar recovered from 13th-century ruins near by.) The runic marks on the stones were simple "building kit" instructions, to tell the masons where each stone should fit.
"When Guedelon is finished," Mr Guyot promised, "the seigneur will arrive on his white charger to take possession of his property." Mr Guyot will be in his mid-seventies by then but he clearly intends that the seigneur should be himself.
And then what? "It would be natural for a new castle to attract a village outside its walls That would take another 50 years. Then we could build a priory. That would take us, not me but whoever follows, up to the end of the 21st century." Mr Guyot gave a sly smile. "After that, maybe a cathedral..."
The Chateau de Guedelon is near the villages of Saint Sauveur-en-Puisaye and Saint-Fargeau in the departement of the Yonne, just to the west of the main A6 autoroute from Paris to Lyons and the south. It is signposted from both villages. In August, it is open every day from 10 am to 7pm. From 1 September to 15 November, it opens from 10am to 5.30pm.)