Sedan Castle was built in 1424 on the site of a Benedictine priory attached to the abbey of Mouzon. The remains of the Saint-Martin church, built by the monks in the 11th century and destroyed in the 19th century, can still be seen in the courtyard. Since the beginning of the 16th century, a chapel reserved for the princely family has occupied the upper part of the ogival pavilion. Ten Gothic columns are placed against the side walls and the apse. The low ribbed vault is in the flamboyant Gothic style. When Prince Henri-Robert de La Marck (1540-1574) converted to Calvinism, the chapel became a Protestant oratory.