The castle was built on a spur overlooking the valley of the River Ailette between Soissons and Laon. A colossal feudal fortress, strongly fortified by the lords of Coucy, who were rich enough in territory and alliances to erect an extraordinary building , it commands the lines of approach from ca. 10 km in every direction. The complex visible today, castle, outer bailey and walled town, was built by Enguerrand III, Lord of Coucy, between 1225 and 1242. At the four corners are cylindrical towers 20 m in diameter (originally 40 m in height). The donjon (keep) was the largest in Europe, measuring 35 meters wide and 55 meters tall. It was renovated by Viollet-le-Duc in the 19th. In 1917, the German army dynamited the keep and the four towers using 28 tons of explosives.