Important remnants of the Classical, Hellenistic and Roman times have been detected on the heights on either side of the Nestos River. The fort of Kalyva stands out among them, on the top of the hill ?Kastro? (627 m. above sea level), 4 km. southeast of the village of Kalyva. Its strategic location ensures the control of a major part of the valley and provides visual contact with other fortresses along the Nestos, such as the fortresses of Aeriko, Paschalia, Dafnonas, Kallithea and Komnena. It was discovered in 1973 by the archaeologist D. Triantaphyllos. The excavations were conducted by D. Triantaphyllos, during the years 1975-1992.

The excavation research showed that the fort was constructed by the Macedonian king Philip II at the second half of the 4th c. BC and that the area was in use till the Justinian?s time (6th c. AD). The enceinte of the fort has a roughly trapezoidal shape with a perimeter of 245 m. It is preserved to a height of 3.50 m. and it is reinforced by two quadrilateral and four circular towers. The most remarkable gate is the one called the ?Priapos Gate?. It was named after the relief of the god found close by and which used to decorate the keystone of the arch. This gate led to an inner courtyard with two ?twin? gates, through which one could enter different parts of the fort. In the wall of the east gate there is a rectangular recess, in which there would have been a statue of the god who protected the gates. On the threshold of the west gate there is a pair of engraved soles of human feet. This pair of human soles had magical and apotropaic meaning and could prevent forced entry of the gates and the seizure of the fort by the enemies.

The most remarkable structure though, discovered almost in the middle of the fort, is a large impressive cistern. The cistern consists a remarkable feat of ancient mechanics and hydraulics and dates back to the time of Philip II. It has the shape of a beehive and it is 12.30 m. deep with a maximum diameter of 8 m. It was constructed with ashlar blocks covered with strong hydraulic plaster, in order to be watertight. Its bottom has a big hollow in the middle as a settling tank. The water capacity of the cistern has been estimated at 389.54 m³. The large pit in which the cistern was built, was created with the extraction of building material for the construction of the fort.