The extensive earthen mounds and walls in southwest Ohio are unlikely a fortress, although they might have been used for social gatherings and religious ceremonies and astronomical viewings.
The site, atop a wooded bluff 235 feet above the Little Miami River in Warren County, was built 2,000 years ago by ancient Indians that archaeologists call Hopewells.
The intricate mounds stretch nearly 3 ½ miles and enclose about 100 acres atop a promontory on the east bank of the river in Washington Township.
The earthen walls are as high as 23 feet and as wide as 68 feet. The walls are divided by 67 crescent-shaped gateways. There are stone pavements in some places.
Some call Fort Ancient Ohio's Stonehenge and it is one of Ohio's top prehistoric sites.
That's actually a pretty good article. Any of you Ohio residents ought to go have a look if you already haven't.