From Ohio History Central
The Fort Ancient Earthworks are a series of earthen embankments that extend for more than three and one half miles around a high bluff along the Little Miami River in southwestern Ohio. Although it is called a "fort," it probably never served as a defensive work. Ditches are located inside the walls rather than outside as might be expected in a fortification. There are more than 60 gateways in the walls, making it difficult to defend the site against enemies.
The earthwork includes two major components called the South Fort and the North Fort. The South Fort is the oldest section. A long, narrow neck referred to as the Middle Fort connects these two parts. The most prominent gateway is located in the northeastern part of the North Fort. Two mounds are located just outside the gateway. These framed the entrance to a set of parallel walls that once extended northeastward for nearly half a mile where they ended in a semi-circular enclosure surrounding a mound.
The Fort Ancient Earthworks is an Ohio Historical Society site open to visitation. It is located seven miles southeast of Lebanon, in Warren County. In addition to the earthworks, the site features a museum with interpretive exhibits about Ohio's ancient past. The Fort Ancient Earthworks site also is a National Historic Landmark.