One of the confounding aspects of the Classical-Hellenistic Corinthia is the dearth of ancient sources that make any clear reference to the countryside. Most of what we can say about the rural Corinthia in Classical antiquity is either drawn exclusively from archaeological sources or teased out of obscure and difficult references in the literary tradition. For some rural fortifications, such as the rubble walls on Oneion, scholars have been able to offer a series of possible events leading to their construction. Stroud (Hesperia 40 (1971), 139-145) argued that these fortifications on Oneion may have been built by Spartans (or their allies) in an effort to block the passage of the Theban general Epaminondas into the Peloponnesus in the early 4th century. Or they may represent the camp of Athenian mercenaries who the Corinthians invited in to fortify particularly vulnerable stretches of their territory. In general, scholars have seen fortifications on the Isthmus proper as being the work of foreign troops who had an interest in preventing the main route into the Peloponnesus from being occupied or easily passed.
The Corinthia, however, is more than the Isthmus as our rural installations around Lychnari bay show. Moreover, the Corinthians would have had significant motivations to protect their own coastline and coastal territory. During the Peloponnesian war, the Athenian raided the unfortified coastal community of Solygeia in the rolling hills immediately south of Mt. Oneion (Thuc. 4. 42-45). Later in Book 8 (10.2-11.2) Thucydides tells how another Athenian fleet landed troops at a the last Saronic harbor before the Epidauria (most recently and plauisibly identified by Dixon (and others) as Korphos). The account (and apparently the battle) between the Athenian fleet and the Peloponnesian forces is a bit confused, but it seems that the Athenian troops on land withdrew owing, perhaps, to Corinthian forces present "in the neighborhood."
The events of the 5th century would have undoubtedly reinforced the vulnerability of the Corinthian coastline to attacks. The rubble fort at Vayia may have been a small camp for a detachment of troops positioned to defend the bay at Lychnari. To speculate: these troops might have been the Athenian mercenaries who were defending the Corinthian countryside in the 4th century (Xen. Hell. 7.4.4).
While such speculating is fun -- it puts a story to otherwise silent stones -- it is hardly conclusive and in no way makes these humble fortification more important. The key significance of the fortification in the neighborhood of Lychnari bay is to demonstrate that the Corinthian countryside was indeed fortified during the Classical to Hellenistic period and that Lychnari bay was worthy of particular attention as manifest in towers and a rubble "fortified camp".
More Corinthian Countryside:
New Research on the Corinthian Countryside: Vayia Microregion
The Corinthian Countryside: The Site of Ano Vayia
The Corinthian Countryside: Distributional Data from the Site of Ano Vayia
The Corinthian Countryside: The Lychnari Tower
The Corinthian Countryside: The Passes of the Eastern Corinthia
The Corinthian Countryside: Classical Vayia