Fort Ancient in southwestern Ohio is a vast Hopewell earthworks complex on a bluff above the Little Miami River, enclosing about 100 acres with roughly three and a half miles of walls, the largest prehistoric hilltop enclosure in the United States. Built in at least three stages between about 200 BC and AD 400, it required enormous labor and careful design. Once thought defensive, it is now understood as a ceremonial, social, economic, and political center, with alignments to solar and lunar events. Studied since the early 1800s, it became Ohio’s first state park in 1891 and is now part of a UNESCO World Heritage Site.