The castle of Coucy's ruins are situated on a promontory which dominates the Oise and the Ailette valleys, between the region of Soissons and the region of Laon. It was constructed at the beginning of the 13th century by Enguerrand of Coucy, called "the Builder". With its huge keep as high as the cathedral Notre-Dame's tower, the castle was part of a group of fortified constructions. Joined to the Crown in the 16th century and demolished by Mazarin in 1652, the fortress was reinforced by Viollet-le-Duc before being seriously damaged during the First World War.