Having worked in and thought about Corinth and the Corinthia for the past decade, it amazes me how little I've thought about its northern neighbor Megara -- despite the fact that we drive through the Megarid on the way from Athens to Corinth.
Philip J. Smith's book, The Archaeology and Epigraphy of Hellenistic and Roman Megaris, Greece. BAR 1762. (Oxford 2008), should make the archaeology of this region more accessible. Of particular interest to me are the fortification across the northern border of Mergara presumably situated to impede progress through the Isthmus. Since scholars have generally regarded the Megarid as "neutral" through most of its history or at least militarily and politically subordinate to her southern neighbor, the fortifications along the border between the Corinthia and Megarid would seem to fit into a more ambiguous category of internal or regional fortifications designed to accomplish interregional goals or highly local ones rather then simply to protect the boundaries of an established state.
Wiseman documented the rubble fortifications at Lysi on Geraneia (20-22) which is the best local comparandum to the rubble fortifications across Mt. Oneion on the southern boundary of the Isthmian plain (see Caraher and Gregory 2006). This rubble fort would seem to have less to do with fortifying the territory of Megara (or the Corinthia) than with blocking traffic across the Isthmus. Smith follows Wiseman in suggesting that Cassander built this fortification in 316 BC when he was most active in the Peloponnesus (Smith, 31). We've argued that our walls on Oneion further south are, in fact, somewhat earlier. Northeast of the site of Lysi, near the site of Paliopyrgos stands the remains of a circular tower which is probably post-antique on account of its mortar and rubble construction. It may sit on an older foundation (or not) which blocked the northern approach to the rubble fort and the passage through Geraneia.
Some 10 km east of the site of Lysi stands another useful comparandum for our recent work on Corinthian fortifications. At the Kaki Skala there is the remains of a round tower and a rectangular fortification at the northern end of the eastern pass from the Megarid into the Corinthia (Smith, 24-25). Measuring about 10 m in diameter, this is roughly the same size as our tower at Lychnari and the combination of a round tower and fortified enclosure parallels nicely the fortifications at Ano Vayia. Smith and Wiseman regard the site as a fortification designed the block the pass south and part of a regional signaling system which could have alerted the polis center of an enemy moving north through the Kaki Skala pass (Smith, 89-92).
Towers or fortifications at the site of Mavrolimini in the northwestern Corinthia complete the fortifications across the southern Megarid and presumably blocked passage through the western side of the the Geraneia ridge. Unfortunately the remains at the site are mostly lost.
There is a nice review of Smith's book here. The only thing that I could add to this is that it is regrettable that the author did not extend his analysis to at least Late Antiquity. The Late Antique Medgarid is poorly documented consisting of a handful of stray finds, graves, and several early Christian basilicas. It seems increasingly common practice to include the 4th-7th century in such sweeping regional studies.