From the record
“The Fort Randall Military Post was established in 1856 to help keep peace on the frontier. It was located on the south side of the Missouri River in South Dakota, just below the present site of the Fort Randall Dam.”
Wikipedia article Fort Randall, CC BY-SA 4.0.
One of 12 National Register listings published here for Gregory County · listed in 1976 · category: District · built in 1856.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service