I wrote about the history in The Treaty of Fort Clark, so I'll just talk about the visit now.
After visiting the Village of Arrow Rock, I followed the lady's advice (referring to the GPS navigator) and took Route 24 westward to Fort Osage National Historic Landmark. Route 24 goes through a number of towns with populations ranging from 50 to 600, following the path of the Missouri River much more closely than I-70. The sites that I'm visiting are along the Missouri River, so that somewhat slower route turned out to be a more direct one.
The staff there were very knowledgeable and happy to help, and I learned a lot while visiting. For a couple of decades following the return of Lewis and Clark to St. Louis, trade with Indians west of the Mississippi River was managed by the U.S. Government, which established a number of trading posts, built forts to protect them and and manned them with soldiers. Trade with indigenous nations was often subsidized, i.e. it was done at a loss to the Government. This was done in order to bring native tribes into the American sphere of influence, as we were competing with the British and Spanish at the time.
There is a visitors center at the entrance, which offers a brief orientation video, and a few exhibits on regional geology, flora and fauna, tribal history and colonial expansion. After exiting the visitor center there's a brief walk to the replica of Fort Osage.
Here's the eastward view from the fort, i.e. looking at the Missouri River downstream
Today's entrance to the fort. When it was in use, the only entrance was from the river, which is on the opposite side of the fort.
Natives brought pelts to this room, were they where appraised, processed and prepared to be shipped.
Directly across the hallway was this room, where the Indians would trade credits (given in return for the furs) for manufactured goods from the east.
At approximately this spot, across the Missouri River from the fort, Lewis and Clark are believed to have camped.
Soldiers and other residents within were greatly outnumbered by the surrounding native tribes, therefore maintaining discipline was of the utmost importance. In the middle of the courtyard stood this whipping post.