It crowns the peak of the naturally fortified hill of Agios Stefanos, above the current residential complex of Megalo Horio. The ancient city was built in the eastern side of the steep hill, invisible from the sea and protected from the winds. Since the 6th century its residences had been built on terraces. They used strong retaining walls and they usually contained a cistern in the basement. The sanctuary of Polias Zeus and Polias Athena were located here as well.
When the Knights of the Order of Saint John occupied the island, they reinforced the fortification of the ancient acropolis by building a powerful castle. The ground plan is in an irregular triangle form and on its east side ruins of a bulky square bastion are preserved. Inside it trails of water tanks are conserved as well as the ruined monastery of the old Agios Taxiarchis (14th century) which is assumed to have been built on the place where the sanctuary of Zues and Athena once was located. The castle visually controlled the bays of Eristos, Agios Antonios and the sailing of the boats to Nisyros.