The Hexamilion ('six mile') Wall is a barrier wall across the Isthmus of Corinth, from the Saronic Gulf to the Gulf of Corinth, designed to defend the Peloponnese against an attack from the north. Literary and archaeological evidence show that the Isthmus was defended at various times in antiquity and plans were made to fortify it during the crisis of the 3rd century.
The devastation of the Peloponnese by Alaric led to the realization of this plan in the early years of the 5th century, at about the same time as construction of the Land Walls of Theodosius II in Constantinople. The wall was 10 Roman feet (nearly 3 m) thick, approximately 8 m high, with towers that were primarily rectangular. A large fortress was located near the former Sanctuary of Poseidon at Isthmia. The fortifications were apparently allowed fall into disrepair and were restored by Justinian. The Hexamilion was defended during the time of the Slavic invasions, but it seems not to have been an effective harrier. Niketas Choniates reports that the isthmus was unsuccessfully defended against the Crusaders in 1205.
In 1415 Manuel II rebuilt the Hexamilion, and during the next half-century it served as the primary defense of the Peloponnese. The Ottomans breached the walls in May of 1423 and the future emperor Constantine XI rebuilt it in 1443; subsequently a prophecy (perhaps inscribed on the wall itself) was circulated to the effect that the Hexamilion would protect a revived empire. The Hexamilion fell again to the Ottoman in 1446 despite a spirited defense. The Venetians restored the fortifications in 1462.
Oxford Dictionary of Byzantium