Kotzinos, the most important Medieval town and commercial harbour of Lemnos, second only to Palaeokastron (Kastro – Myrina), is located at the northern side of the island, close to the ancient Hephaestia, at the inner spot of the Bournias Bay. The gradual degeneration of the harbour of Hephaestia, mainly due to accumulation of silt during the Late Roman period, created the need for a bigger and safer harbour like Kotzinos, which thus was founded in the Middle Byzantine years. Today Kotsinas is a small village of fishermen in which the ruins of the Medieval walls represent the only witness to the second most important town of Lemnos during the Middle Ages. The financing of a peliminary research in the area, coupled with the “European” interest in fortified settlements that are endangered to vanish, offered the opportunity for an in situ investigation and study of the remaining evidence. The combination of the archaeological and historical data proves that the Medieval fortress Kotzinos -and its broader region- has been of crucial importance, since it could, and still can control the Hellespont straits, the major commercial station of the sea-route to Constantinople. Its significance is further ascertained by the continuous conflict mainly between Byzantine and Venetian merchants for the domination of the area as well as by the strong interest of the monasteries of the Mount Athos in their properties on the island, the metochia that occupy the most fertile land of Lemnos.