A number of forts were built in NW Iowa in 1862 following the "Great Sioux Uprising" in Minnesota. The Northern Border Brigade was organized which kept able bodied men in Iowa and prevented them from going south to fight the Confederates. The threat of Indian attacks in Iowa did not materialize.
Fort Peterson was a triangle shaped fort with this blockhouse on one corner. It was eventually dismantled and later rebuilt as a residence with some changes in the original design. The gun ports are visible in these photos. There is also a plaque here at the actual site of the fort in Peterson. It is off Highway 10 at Second Street and Park Street just east of the business district.