Asen's Fortress is a medieval fortress, named after tsar Ivan Asen II, situated in the Rhodope Mountains on the top of an enormous rock on the left bank of the Assenitsa River, 3 km south of Assenovgrad. The fortress has been a busy place during both Roman and early Byzantine epochs. After the Ottoman invasion of the 14th century the fortress lost its strategic position as a frontier stronghold. The best-preserved building in the fortress is the Church of the Holy Mother of God (St. Mary of Petrich) which dates from 12 century.