The Late Antiquity and medieval fortress near the town of Koshnitsa, Smolyan District, known as Kaleto, is located on a 1006-meter mount of marble and limestone. It is connected with a cave leading into the valley of the Arda River. Archaeological excavations have found evidence that the cave as used as a prehistoric sanctuary while the mount itself was fortified as early as the 5th-4thmillennium BC (during the Chalcolithic (also known as Aeneolithic or Copper Age)). Its site has traces of prehistoric human presence from the Late Chalcolithic, the Bronze Age, the Early Iron Age (11th-6th century AD), and the period of Ancient Thrce and Ancient Rome.
The fortress near Koshnitsa was in use in the Late Antiquity and Early Byzantine period – in the 6th century – and again during the period of the Byzantine occupation of Bulgaria (1018-1185 AD) and the Second Bulgarian Empire (1185-1396 AD), in the 11th-13th century. During the Late Antiquity, the fortress near Koshnitsa guarded major routes within the Rhodope Mountains, namely, the transitional route in the mountains linking the so called Diagonal Roman Road (Via Militaris) and the Roman Road running along the Aegean coast (Vid Egnatia). It has been by archaeologists from the Smolyan Regional Museum of History.