Those of us that have been to Ft. Ancient have little doubt it was a hill top fort by the Hopewell People. Who did they fear? South Americans come to mind as well as some southern United States cultures. I once read where the Miami Indians (historical) talked of the ancients ones being giants (Adena culture) which is before Hopewell. The few grave-sites excavated by archaeologist suggest this as being true, not giants but somewhere near seven foot.
Not sure about Fort Ancient, but most like Cahokia look to be ceremonial centers. Fortifications are usually compact rather than spread out and have post holes from a palisade like, say, Iroquoian forts of the immediate pre-Columbian era.
It’s fascinating because it sure does look like American Indian civilization was actually declining when Europeans first came. Cahokia was flourishing...what around 1200-1300...and after that started to disintegrate. DeSoto just barely caught the tail end of Mississippian cultural complex on his march through the South.
Maybe as the civilization was declining, the Cahokians/Fort Ancient people were being attacked by other tribes the way that Rome was being picked apart by the Vandals and Goths. And that’s what got remembered in a garbled way as fortifications.