Above is a scene from "How the West Was Won" with a Pony Express rider charging into a fort, switching horses and riding out. The fort was built on a hill along the Wildlife Loop Road in Custer State Park.
One of those corner blockhouses was left standing and I remember climbing up into it even into my high school years around 1979 or '80. Unfortunately it is gone now.
Also, here is a video trailer for the movie How the West Was Won. It includes several of the South Dakota scenes, including a quick clip of men shooting off a moving train at Indians that I don't remember seeing before.