The strategic fortress of Isthmia, taking advantage of favorable terrain, was located to the southern side of the Hexamilion wall, north-east of the Poseidon Sanctuary.

The blocks on the northern facade were larger and coalesced with more carefully implemented edges, while the southern face was conceived of smaller stones set in mortar.

Some structures were incorporated into the wall directly (as was the temple of Poseidon at Isthmia) whereas some were burned into lime (as was the sanctuary of Hera at Perachora, as well as much of the ancient statuary of Corinth).

[11] As part of his repairs to the wall, Justinian established a professional military garrison within the Fortress of Isthmia, which replaced the local farmers who previously manned it.

To bolster supplies, the soldiers produced some of their own food through farming south of the Hexamilion, although major aid came also from local farmers, merchants, artisans, and workmen, including from other nearby towns, such as Corinth.

One such piece of evidence was the discovery of graffiti scratched onto the rear face just west of Tower 15, This was undoubtedly made by individuals associated with the wall's initial construction or repair, as the etching occurred before the mortar had time to harden.

The image depicts two galleys and a different kind of vessel seen as a boarding device suggesting naval combat and the notion of the Hexamlion’s lack of defense from seaborne threats.

[12] The construction of one of the graves resulted in the removal of the bottom tread of the staircase, undermining the functionality of one of the most strategically important points of defense along the Hexamilion wall.

The graves were created over a span of many decades and contained women and children, suggesting that soon after its initial construction, the Fortress' maintenance passed over to local residents.

[7] During the later sixth and early seventh century, both the Northeast and South Gates of the Fortress were sealed with thick walls, effectively blocking the busy roadways to Athens, Corinth, and Epidaurus.

The re-fortification of the Isthmus in an effort to counter the invasion of the Ottoman Turks in 1415 CE during Emperor Manuel II's reign led to an open revolt among the local population, which was put down by force.

[13] Manuel II saw the opposition as open resistance to the reinstatement of imperial control, whereas Chrysoloras documents a growing local frustration with the continuous funding and building of the wall.

In 1415, Byzantine emperor Manuel II personally supervised repairs over a period of forty days, but the rigorous demands of this effort caused unrest among local elites.

[15] Constantine Palaiologos, who was Despot of Morea before his accession to the throne of the Byzantine empire, and his brother Thomas restored the wall again in 1444, but the Ottomans breached it in 1446 and again in October 1452.

Murad II, commander of a Turkish army said to have consisted of 50,000 to 60,000 men, supposedly lined the entirety of the wall with heavy artillery of long cannons (new weapons at the time), siege engines and scaling ladders.