The Justinian or Hexamilion wall was one of the largest defencive structures of all time, used to protect Peloponnese against invaders. It extended through the Isthmus, from the shores of Saronic to the Corinthian gulf. It was connected to the fortress at Isthmia and had 67 towers.

The wall was built in the Byzantine period, under the emperor Justinian, and repairs were undertaken by Manuel II Palaiologos. The wall protected Peloponnese for over a millennium, being in constant active use. The Turks caused massive damage in 1431 and emperor Constantine XI Palaiologos repaired it in 1443. The walls were conquered by the Turks in 1446, though. In the end of the 15th century, the Venetians repaired the wall, albeit poorly.

It had a length of 7.5 kilometres and a width of 3 meters. In its construction, ancient materials were also used.

Today, a small part of it remains in a spot about 2 kilometres from the Corinthian Isthmus, on the right side of the old national highway.