The ruins that can be seen near the settlement of Porto Lagos belong to the Byzantine fortification of Poroi. Although only 170 meters of the enclosure survive today, it is estimated that its extent was much greater. The towers and stairs leading to the perimeter wall are visible.
The name derives from the passage from the Thracian Sea to Lake Vistonida. This was the reason why it developed significantly as it played an important role commercially and strategically. The foundation of Poroi dates back to the 9th or 10th century. It was destroyed by the Bulgarian Tsar Ivan I known as Kalogiannis or Skylogiannis. The final abandonment of the settlement came in the 14th century.