Building a fortification as if you were in the 13th century!

How medieval builders managed to build fortifications? This has been the goal of an intersting project led since 1997 by Michel Guyot to construct "château de Guédelon", which is being built following the model of a 13th century French medieval fortification. The project is supervised carefully by a scientific committee containing experts in medieval French architecture. A book has already been published about the scientific aspects of the project.
Here is the various phases of the construction from 1997 to 2006. The project will be open to the public by 2025.