The Asen’s Fortress is located in the Asenovgrad region of Bulgaria along the Asenitsa River. Because of the steep cliffs, the Asen Fortress is impregnable on three sides, making it the ideal location to protect the strategically important entrance to the pass connecting Thrace with Northern Greece and the Aegean Sea. The Thracians initially built the Fortress in the 5th century, and in the 9th century, the Roman Emperor Justinian rebuilt it to protect against Slavic invasions. In the 10th century, the Byzantines occupied the area and built the first fortress tower. Later in 1204, the Crusaders under René du Troisoccupied the Asen Fortress, rebuilding it and expanding it.Since so many people occupied the Asen Fortress, it has a rich archaeological history. Findings include the foundations of a feudal castle, complete with living quarters, a church for the castle lord, and a fortress tower.