The Octagon Earthworks consist of a series of earthen walls forming a massive octagon attached by an enclosed causeway to a circle. The octagon has eight wall segments separated by open gateways, and just within each gateway sits a small oblong mound. These mounds form two squares within the enclosure. The circle is nearly perfect in shape and is 1,054 feet in diameter, a measurement often repeated across Hopewell sites. At the far end of the circle stands a large earth and stone mound called Observatory Mound, marking the base of the earthwork's central axis.