In Guédelon in northern Burgundy, 45 craftsmen are building a medieval castle. Construction is expected to take 26 years. And half the work has just been completed.Working conditions are like those of 800 years ago. Here there are no cranes or cement mixers. Every nail is forged by hand. The unit of measurement is the ell. Here archaeologists can experience the actual conditions under which a medieval groined vault was built. Guédelon is a tourist attraction. And the one concession to modern security regulations -- hard hats on the"medieval"workers -- doesn't bother the visitors.