The Isthmus of Corinth is α narrow strip of land between Central Greece and the Peloponnese, and it is the only entrance from the north into the southernmost part of the Greek mainland. For that reason, it played an important part in the history of the area. Again and again, the Peloponnesians built and repaired α line of walls across the narrow neck of land, to the south of the modern canal. In recent years, the foundations of a Cyclopean wall have been discovered south of the sanctuary of Poseidon; this must have been constructed in the late Mycenean period in α vain attempt to stem the Dorian invasion.

Much of the Hexamilian Wall, built in the time of the Emperor Justinian of Byzantium, has survived. It was subsequently reconstructed twice - in 1415 by Manheul ΙΙ Palaeologus and in 1443 by Constantine Palaeologus ­before the Turks finally broke through it in 1446. The wall is 7,300 meters long, with α thickness of 3 m., 153 towers and α large tort at either end. Much ancient material from the sanctuary of Poseidon was incorporated into the wall - so much, indeed, that it was believed for many centuries that it has been a Classical structure.