The hilltop includes the Acropolis, measuring 190 x 80 meters, which is enclosed by a circuit wall with towers. The east side is preserved to a great height; it is reinforced by four towers and has a sally port. The towers are constructed in the isodomic style, whereas the walls feature trapezoidal, polygonal, or irregular masonry styles.

The most impressive feature of the fortress is its southeast tower. It is square (sides 8.8m.) and some 20 meters in height. Joist-holes on the interior for supporting wooden floor beams provide evidence for three storeys. On the three vulnerable sides there are archers? slots and in the top storey, which would have had a double pitched roof, three catapult windows on both the south and north sides reinforce the defensive character of the tower.

The Acropolis was connected to the harbour by long walls, of which only the northern one is visible today. It was reinforced by eight towers and bastions and had at least two gates.

There is evidence at Aigosthena for the cult of the hero and soothsayer Melampus, whose sanctuary is thought to be below the Acropolis in the area inside the long walls. Habitation during the Early Christian period is attested by the 5th century A.D. five-aisled basilica constructed of ancient building material which lies adjacent to the modern church of Panaghia. In Post-Byzantine times, the Acropolis area held a monastery, from which the ruins of monks? cells and the katholikon (main church), the small church of Aghios Georgios, are preserved today.

In 1981, a powerful earthquake caused the partial collapse of the SE tower, which is currently being restored by the 3rd EPCA with funds from the National Strategic Reference Framework (NSRF).