Köprügören Castle, locally known as Köprügören Kalesi or Osnak Kalesi, lies on a rock above the village of Köprügören, in the valley of the river Çoruh, in the province of Artvin in Turkey. The name Osnak is the old name of the village.
I was not able to find any historical information about Köprügören Castle, other than that it was probably built during the Middle Ages because of its walls being built out of rubble masonry. So, if you know more; please mail me.
The castle consisted of a square keep on a separate rock and a bailey on another rock. Of this former bailey nothing remains.
The site of Köprügören Castle is freely accessible. The path that would enable you to enter the tower was a little to steep and slippery for my taste, however. Not a very spectacular castle. Situated a little more down stream than nearby Yokuşlu Castle, I am afraid it will be more affected by the future reservoir of the Yusufeli Dam, which is being built at present.