Union Col. James A. Mulligan of the 23rd Illinois Infantry supervised the construction of Fort Mulligan between August and December 1863. Known locally as Fort Hill, it protected the South Branch Valley and its Unionist residents and also served as an auxiliary depot for Federal camps occupied in October 1861 and May-June 1862. Federal troops manned the fort until 1864. An earthen fortification, its walls were lined with timber for additional strength, and abatis outside the walls helped defend it against attacking infantrymen. The fort had at least three entrances, seven artillery emplacements, and four bombproofs that protected arms, artillery shells, and gunpowder from bombardments. Union Col. Joseph Thoburn ordered its evacuation on January 31, 1864, when Confederate Gen. Jubal A. Early’s army threatened an attack, and Early’s men demolished the empty works. Although engagements occurred in the area for the rest of the war, the site was never again occupied.
