The fortress was built in Late Antiquity as part of a series of strongholds that guarded the coastal road leading to Constantinople. It is first attested during the reign of Justinian I (527–565). Kyklobion was used as the landing-site of the Arab armies on both of their assaults on Constantinople, in 674 and in 717. In the early 8th century, the iconodule Saint Hilarion was kept prisoner in the local monastery on the orders of Emperor Leo V the Armenian (r. 813–820). The site is again, and for the last time in Byzantine times, mentioned in a property deed of 1388.