Founded in 1995, the Guedelon Project is an exercise in “experimental archeology.” Workers are building a castle to the specifications of French nobleman Gilbert Courtenay who started construction in 1253 before he went off to the Crusades leaving his wife to supervise the work.

Unlike the process of restoration, which offers workers direct evidence of what was there and, often, how it was built, the Guedelon Project requires workers to configure it all, from basic tools to modes of craftsmanship. Along the way, Guedelon artisans are providing valuable information to craftsmen restoring the Cathedral of Notre Dame in Paris after a fire nearly destroyed it in 2019.

For example, there’s Stephane Boudy, one of a small team of carpenters working at Guedelon since 1999. He has learned that splitting wood by hand preserves the heart of the resulting beam for strength and resilience. “We have 25 years experience of cutting, squaring, and hewing wood by hand,“ the Guedelon website quoted him as saying. “We’ve been doing it every day for 25 years…If this place didn’t exist, perhaps experts would have said: ‘No, it’s not possible to reproduce the roof of Notre Dame.’ it is. This isn’t just nostalgia. If Notre Dame’s roof lasted 800 years, it’s because of this. There’s no heart in sawmill wood.”

Watch the Guedelon Project at work.