Fort D is Missouri's sole surviving urban Civil War fort

Construction of Fort D occurred in summer and fall 1861 at the same time as the other forts. The layout of Fort D is a redan, or a V-shaped projection pointed in the direction of an expected attack. The fort is the smallest of the four Cape Girardeau forts. The plan map for Fort D matches the current footprint of the fort. The fort is a little over 400 feet across, and about 200 feet from the apex of the redan to the rear. The plan shows a palisade fence with opening in the back, toward the Mississippi. This fence ran on a line through the front part of the 1937 blockhouse.

Among the troops assigned to the fort early in the war were members of Company G, 17th Illinois Infantry. A member of this company, Benjamin J. Radford, described three 32-pounder seacoast guns placed en-barbette, allowing them to swivel from side to side. These guns were at the vertices of the nearly-triangular fort, and ramps led to the guns. In between, at angles between the larger guns, were two 24-pounder guns, likely mounted on wheeled siege carriages for mobility. The powder magazine was near the northwest wall at the edge of the fort, not centrally located as many assumed.

Although not obvious today, Fort D and the other forts were at sites providing a panoramic view of approaches to the city from the north, west and south. All but Fort A were actually outside town in 1861, and only scattered farm houses or estates were within view of the sites. Today the views are difficult to discern because of residential development and planting of street trees. The view from Fort D commanded the approach to the city along Benton Road, roughly present-day South Sprigg Street. This approach to the town was far less useful for attackers than other approaches such as Bloomfield and Jackson Roads, so the cannons never fired except for drill.

Mary Hunter Giboney owned the site, given to her by her father Andrew Giboney. Mary wed Louis Houck in 1873, and she was active in the local Chapter of the United Daughters of the Confederacy.

Efforts to save Fort D began with efforts of local civic groups and veterans in 1911, when Houcks began developing the surrounding area.

The site stayed in Mary Houck's ownership until sold to the Louis K. Juden Post of the American Legion in 1935. Development with WPA funding began, including repair of the walls and construction of the building. The Legion used the blockhouse as a museum and meeting place, before deeding the site to representatives of the Girl Scouts in 1945. That group later deeded it to the city years. The city used the building for various purposes until the late 1960s. Local volunteers stepped forward to clean up and interpret the site in 2005. Renewed focus on Fort D culminated in repair of the building's roof and listing of the fort in the National Register of Historic places in 2019.