Double Ditch Indian Village Historic Site, about 10 km north of Bismarck along the Missouri River, preserves the earthworks of a large Mandan village occupied from around 1490 to 1785. The site takes its name from the two concentric ditches — originally defensive moats — that encircled the village. The raised circular depressions of the earth lodges are still clearly visible on the river bluff, along with the midden mounds and the outermost defensive earthworks.
The site is undeveloped and unmarked beyond basic signage — you walk across open grassland with the river below and the earthwork shapes rising naturally from the ground. It requires some historical imagination, but that is part of its power: this is what remains of a community of several hundred people that vanished from European-introduced disease before it ever encountered Europeans directly. The Keelboat Park mentioned in Atlas Obscura entries for the area sits nearby, with a replica of the Lewis and Clark keelboat. Free access, open dawn to dusk.