"Fort Johnson was a U.S. Army post built on bluffs overlooking the Mississippi River in modern-day Warsaw, Illinois during the War of 1812. The fort was established in September 1814 by Major Zachary Taylor, future U.S. president." "Because of the chaotic situation along the Mississippi River frontier, the fort was abandoned in late October 1814 after provisions ran out. The company retreated to Cap au Gris near St. Louis." Source: Wikipedia
The Quincy Herald Whig had an article (reprinted at the Illinois State Archaeological Survey website) about the fort at Warsaw, Illinois:"It wasn't their intent to leave a lot behind, but it's amazing what is here given the short length of their stay." "They didn't want them to fall into the wrong hands," Nolan said." "The wrong hands, the British and the Sauk Indians, were never far away."
"In between St. Louis, which was a hub, and Fort Dearborn in Chicago, it was pretty much no man's land. Other than Fort Madison, we basically have the only story to be told here."
The Callaway Family Association blog has a copy of the Callaway map of Fort Johnson (thought to be the sole drawing of the fort).