Site A is located to the south of the National Highway from Athens to Lebadeia at the 91 km mark. It lies on the foot of a hill looking across the present-day Kazarma area. Ploughed up, worked stones were already noted by nineteenth-century travelers, encouraging a connection between the site and the famous sanctuary known from ancient literary sources.

In 1971, the Ninth Department of Prehistoric and Classical Antiquities conducted a brief but systematic investigation of the roughly 0.6-hectare area, during which the foundations of two structures were unearthed: a badly preserved rectangular building, and to its southwest, a larger hall with an internal colonnade, both apparently dating to the sixth century BCE based on masonry techniques used in their construction. According to the original interpretation the smaller structure was seen as the Temple of Poseidon and the hall as a meeting place (bouleuterion). So far the Onchestos Excavation Project has focused on the large hall and a smaller annex to its west.