The cyclopean wall of the late Mycenaean era is associated with fortifications made in anticipation of an invasion (13th - 12th century BC). The Isthmian wall, with a fortress to the E. of the sanctuary of Isthmia, the so-called "Hexamillion" in medieval times, which was originally built at the beginning of the 5th century. A.D. with building material from the sanctuary of Isthmia, it was rebuilt in a new, more fortified form by Justinian I (6th century).
Source: MINISTRY OF AEGEAN
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