Asen’s Fortress is built on a cliff overlooking the Asenitsa River in Bulgaria’s Rhodope Mountains, and is impregnable on three sides. It isn’t surprising therefore that there has been a fortification here since the time of the Thracians, who fortified it in the 5th century BC. The fortress was rebuilt during the time of the Roman Emperor Justinian as one of a series of fortresses erected to defend the Empire against invasions by Slavic tribes.