The Rion Fortress (also Rio Fortress or Rhion Fortress) is a historic fortification in Rion, Achaea, Greece, at the entrance to the Gulf of Corinth. Built in 1499 by the Ottoman Sultan Bayezid II, along with the Antirrio Fortress, to protect the gulf from invaders, the fortress was used as a prison after the Greek Revolution and during the German occupation, but today it is a venue for cultural events, particularly concerts in the summer.