The Donjon de Niort or Chateau de Niort is a medieval castle in the French town of Niort in the departement of Deux-Sevres. It consists of two square towers, linked by a 15th century building and dominates the Sevre Niortaise valley. The two donjons are the only remaining part of the castle. The castle was started by Henry II Plantagenet and completed by Richard the Lionheart. It was defended by a rectangular curtain wall and was damaged during the Wars of Religion. From the 18th century, the castle served as a prison.