Fort Ancient (22 WA 2) is a collection of Native American earthworks located in Washington Township, Warren County, Ohio, along the eastern shore of the Little Miami River about seven miles (11 km) southeast of Lebanon on State Route 350. The site is the largest prehistoric hilltop enclosure in the United States with three and one-half miles (18,000 ft) of walls in a 100-acre (0.40 km2) complex. Built by the Hopewell peoples, who lived in the area from the 1st century BC to the 6th century AD, the site is situated on a wooded bluff 270 feet (82 m) above the Little Miami. It is the namesake of a culture known as Fort Ancient, who lived near the complex long after it was constructed.
 
Maintained as a state historical park, the site was designated a National Historic Landmark for its significance. In addition, this is part of the Hopewell Ceremonial Earthworks, one of 14 sites nominated in January 2008 by the U.S. Department of the Interior for potential submission by the United States to the UNESCO World Heritage List.
 
Museum
 
The site now includes a 9,000-square-foot (840 m2) museum covering 1500 years of American Indian heritage in the Ohio Valley. Topics include North America's earliest people, the development of agriculture, and the impact of Europeans who migrated to the area and came into conflict with its inhabitants. The Museum also contains a classroom, a research area, and a gift shop. The site is open to the public from April through October; group tours are available by appointment on a year-round basis.