In the summer of 1861, four forts were built around the strategic city of Cape Girardeau. Fort D was designed by German-American engineers. The forts were built by soldiers, under the direction of Lt. John Wesley Powell; he later gained fame as the explorer of the Grand Canyon.
Fort D featured as many as five cannons, the largest of which could fire a 32-pound cannon ball. The fort was manned throughout the Civil War.
Of the four earthen forts, only Fort D remains. It is located four blocks south of the intersection of Route 74 and Sprigg Street.