The massive fortifications built along the isthmus in the 5th century AD were designed to protect the Peloponnese from an invasion from the north by the Visigoths. At Isthmia, these include the Byzantine Fortress and the Hexamilion Wall.
The fortress at Isthmia was built in the 5th century AD and incorporated a Roman monumental arch. This, then, became the northeast gate to the fortress, and thus the grand entrance into what had been the Sanctuary of Poseidon, as well as the entire Peloponnese. The structure lies 150 m. to the east of the Roman Bath and projects southward from the Hexamilion Wall.
Excavation in the northern areas of the Fortress has uncovered many grave sites, sometimes containing several burials, and it appears that life continued in the fortress even when there was no threat of invasion. Excavation of these grave sites suggest that entire families lived there in times of peace. Throughout its history, the fortress was refurbished and used by many different forces including the Byzantines, the Venetians, the Franks and the Ottoman Turks; and it remained in use until the 17th century.
The Hexamilion (six-mile) wall stretches across the entire isthmus of Greece. The Hexamilion required an enormous quantity of stone to construct, and many no-longer used buildings of the Sanctuary of Poseidon, including the Temple and surrounding constructions, were plundered for stone. The temple itself was torn down to its foundations. The wall was constructed of large ashlar masonry blocks, mortar, and rubble, and still survives in many parts today. Overall, there were 153 towers along its length. Literary sources tell us that several walls were built across the isthmus throughout antiquity, to protect the Peloponnese against invasion.
Cataloging Hexamilion spolia, 1997
The Hexamilion and the Fortress are continued points of interest and study for OSU Excavations at Isthmia. Currently, the Ohio State University Excavations at Isthmia staff are conducting a study of the spolia within the Hexamilion in an attempt to locate, identify, and record blocks which originated from the structures within the sanctuary. During the fifth century AD, when the fortifications were being constructed, the builders made use of the ready supply of cut stone and marble that had originally been part of Temple, Bath, Theatre, and athletic structures, but had since fallen into disuse. This work is ongoing and expected to continue through the 2001 field season.