The ferruginous sandstone of Puisaye is widely used as a bedrock in medieval constructions. According to the local tradition, it would help fight against the undesirable effects of humidity. Its low porosity indeed avoids damp rising by capillarity in the masonry and the limestone blocks used in the higher parts of the buildings. This specific quality as well as the significant mechanical resistance and the frank facies of this rock fully justify its use on the reconstruction site of a medieval castle in Guédelon in the Yonne department.