Patmos
Ardino, Bulgaria
Patmos (Bulgarian) is a medieval fortress in the Rhodope Mountains of southern Bulgaria rising over a promontory near the confluence of the rivers Arda and Borovitsa. A few kilometers to the west upstream the valley of the Arda are located the ruins of the fortress of Krivus.
Fort data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).