The castle Frourio is on top of the hill Kales (Castle) in an altitude of 627 m, 4 km SE of the abandoned village Kalivas. The location is strategic because a large part of the valley of Nestos is controlled. There is also a visual contact with other forts in the nearby area. It was manufactured by Philip B "of Macedonia around 340 BC in the position of the earlier Thracian establishment of the Early Iron Age (9th-8th century BC). It was used by Philip E, the Persians, the Romans and Justinian.