Strabo mentions that Eressos "was founded on a hill and lied by the sea". Despite the insufficient archaeological and stratigraphical evidence, there is no reason to doubt that Eressos was founded in the Late Bronze Age, after the arrival of the Aeoleis, along with all the other towns of Lesbos.

The acropolis of Eressos is located on a naturally defensive hill on the SW coast of the island, and in antiquity it was called "Mastos". According to Koldewey, the total length of theenceinte was 1 km and the enclosed area was about 4,5 hectars. Several parts of the ancient fortification wall are preserved near the foot of the hill. It is built of trachite in the lesbian masonry but several sections are built in a different manner. The whole structure is dated to the Archaic period. Gates and external towers were also distinguished by Koldewey. In the Hellenistic period the settlement was extended to the south of the original enceinte, in the area of the harbour. Another 1,5 hectars were added to the habitation area and a new wall was constructed, this time in the isodomic system.

Remains of a medieval castle are also preserved on top of the hill. It was built in the period of the Gateluzzi and remained in use until the capture of Lesbos by the Turks, in 1462.

The site has not yet been systematically excavated. In the 1880's, during his tour on the island, Koldewey visited the acropolis and made drawings and descriptions of the visible architectural remains.