The Old Stone Fort is a prehistoric Native American mound site whose long wall-like mounds form a hilltop enclosure, a typical though not plentiful kind of construction in the Woodland Cultural Tradition about 2000 years ago. Early settlers mistakenly called sites of this kind forts, but hilltop enclosures appear to have marked the limits or boundaries of special places where people probably gathered for ceremonial or ritual purposes rather than lived. The enclosure’s walls were maintained and modified over a span of 500 years, showing that the site remained significant for many generations. The long mounds are mostly stone inside, built with two parallel stone cores enclosing an earthen fill and probably capped with additional earth, though the stone cores are no longer visible because of forest humus and soil spillover from the original cap. Before the mounds were raised, the planned path was cleared, leveled, and newly surfaced, and a trench was cut along the length of the construction to separate the two stone cores and define the earthen fill between them, often lined with vertically placed slabs of black shale. Middle Woodland sites of this kind share construction patterns sometimes called an architectural grammar, and closely similar enclosures appear even hundreds of miles apart, especially in what is now southern Ohio, as part of the far-reaching architectural and cultural influences known as the Hopewell phenomenon. The ceremonial entranceway at the Old Stone Fort begins with two conical mounds flanking the pathway and attached to the long perimeter mounds; behind them, on the sides facing the two rivers, was once a ditch about 6 to 8 feet deep. In line with the conical mounds but never connected to them are two in-turned parallel mounds, one of which makes a 90 degree turn at the back of the entrance complex, forming an in-turned box shape with open sides facing the sunrise. The two long parallel mounds are oriented within one degree of the summer solstice sunrise, suggesting a possible use in seasonal ceremony.