The citadel or the acropolis of Ayasuluk was built by the Byzantines, although the exact date of construction is unknown. Due to finds of Mycenaean pottery and other artifacts here, it is thought that the acropolis was used by indigenous Anatolian people before the arrival of the Ionian Greeks around 1000 BC.

The Turks rebuilt and added to the citadel, which has 15 watch towers and contains numerous cisterns, the remains of a Byzantine church and a small mosque. The church is visible to the left in the photo above, and the mosque and its minaret in the centre.

Immediately below the fortress runs the wall which surrounds Saint John's Basilica (see gallery 1, pages 5-13) on the south side of the hill. The Turkish name Ayasuluk (Ottoman Turkish, Ayasluğ) is a corruption of of the Greek name of Saint John, Agios Ioannes Theologos (Ἅγίος Ἰωάννης Θεολόγος).

Because the fortress was until recently used by the Turkish military, it was inaccessible to the public for many years. Following renovation and restoration work it was opened to the public, but then closed again, apparently because of unsafe structures.