The earliest archaeological findings date from the time of the Thracians, the area of the fortress being also inhabited during the Ancient Roman and Early Byzantine period. The fortress gained importance in the Middle Ages, first mentioned in the statute of the Bachkovo Monastery as Petrich in the 11th century. The fortress was conquered by the armies of the Third Crusade. It was considerably renovated in the 13th century (more precisely 1231) during the rule of Bulgarian tsar Ivan Asen II to serve as a border fortification against Latin raids, as evidenced by an eight-line wall inscription. The foundations of fortified walls—the outer ones being 2.9 metres (9.5 ft) thick and preserved up to a height of 3 metres (10 ft), originally 9 to 12 metres (30 to 39 ft) high—a feudal castle, 30 rooms and 3 water repositories have been excavated from this period.