Maybe something in the 1800s built by Mormons for some reason?
Humm. Back further.... "There were giants in those days....."
Grizzly bears, Saber Toothed Tigers, and Woolly Mammoths still lived. Good to have Fortress to stay in at night? An ancient Riesenheim?
Maybe they herded Mammoths or mastadons and needed a Fold, a livestock compound? (Fanciful...unlikely Just having fun. )
If they are serious about looking into this they need to start excavating the site to see what they can find. Unless something is found Its pretty much just speculation.