The Hexamillion wall was created to fortify the only land route into the Peloponnese peninsula ~ but throughout it's history (450-1460 AD) it failed to prevent multiple invasions, acting only as a deterrent.
It has been noted that a fortification of the Isthmus was of no utility without control of the ocean.
Was it a lack of control of the surrounding ocean that prevented the wall from being successful during it's history? Or perhaps the construction of the wall itself that allowed it to be easily breached? Maybe a series of ill-led, under strength defenders?