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.