Krivus
Kardzhali, Bulgaria · built 1000
Krivus (Bulgarian) is a medieval fortress in the Rhodope Mountains of southern Bulgaria rising over a high hill on the meanders of the river Arda. Constructed in the 10th century, the fortress was controlled by the Byzantine and the Bulgarian Empires until the region was conquered by the Ottoman Turks in the second half of the 14th century. Several kilometers to the east downstream the valley of the Arda are located the ruins of the Patmos Fortress, while a few kilometers south upstream is Dyavolski most bridge.
Fort data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).