King Asen's Fortress  stands at a very strategic location -  the beginning of the passage connecting Thrace with the Aegean Sea, along River Chaya’s defile, 2 km. from Asenovgrad town. “Perched” on an inaccessible rock, the fortress provided effective natural protection and used to be an important defensive point for centuries. Studies show that a Thracian fortress, dating back to the beginning of the Late Iron Age, existed at the same location. After the establishment of the Eastern Roman Empire, the fortress fell within its territory. In the 9^th century AD the castle was reconstructed to guard the Byzantine frontiers from Slavic invasions. During the entire 11^th century the castle had its own army and administration. The process of its integration with the nearby village resulted in the formation of the medieval city of Stenimachos, known under this name since the beginning of the 13^th century.

The name of the fortress is associated with Tsar Ivan Asen II, who undertook the greatest reconstruction and expansion of the fortress. In that period, the monarch built a castle, located at the highest and most fortified part of the fortress. The castle complex consists of both residential and commercial buildings, a fortress tower, reservoirs and others. The exquisite fortress church "St. Mary of Petrich "was built on a rock platform at the foot of the feudal castle.

During the Ottoman invasion, the conquered fortress served as a stronghold of the empire. During the strife following the death of Sultan Bayezid in 1402, one of his sons - Mussa – tried to barricade in the well-preserved fortress, but didn’t withstand the siege and eventually surrendered to his brother Suleiman. After the events, the fortress was ruined to the ground. Only the church "St. Mary of Petrich”, as we know it nowadays, remained intact. The church is a cultural monument of national importance. It is restored and re-consecrated and at present it's an active temple again.