Castles National Landmark is located in the Slim Buttes land unit in South Dakota. The Castles are a massive sandstone remnant which originated as a volcanic ash deposit and resemble a medieval castle. This natural wonder also serves as a habitat for one of North America's largest populations of Merlin falcons. One of two National Natural Landmarks on the Sioux R.D.
Castles National Landmark site is not in Montana, as stated in the description it is in South Dakota. Forest service has the site listed wrong on its map on their website. Site is next to Reva Gap Campground in South Dakota https://thedyrt.com/camping/south-dakota/south-dakota-reva-gap-campground. The Landmark it self does not have a campground. The site shown on the map in Montana is Becon Rind Creek Trailhead.