The Fort Union Trading Post National Historic Site wasn’t a fort, but was the longest-lasting fur trading post in the west (39 years). It was built on the land of the Assiniboine. The trading post was dismantled so the timber could be repurposed, but a century later it was made a historic site, excavated and rebuilt to replicate the site from the 1850s. The Missouri River used to flow right where the video at the end is taken, but now sits about 700 yards away. This historic site spans 2 time zones in 4 counties across 2 states.
