Federal or Confederate troops occupied this strategic hill and the surrounding area near Petersburg, West Virginia as early as August 1861. The fort was constructed from August through December of 1863, by Union troops under the command of Col. James A. Mulligan, from Chicago, Illinois. Infantry, cavalry and artillery soldiers from West Virginia, Pennsylvania, Maryland and Illinois carried out the labor. Constructed of dirt earthworks, the inner walls were lined with timber. A defensive barrier of abatis was placed on the outer entrenchment walls to prevent a major assault. Union forces used the fort to quarter troops patrolling against Confederate raids on the Baltimore and Ohio Railroad to the north, and as a "jumping-off" point for their own raids further south. The intersection of the road network at Petersburg and Moorefield and the support of Unionist groups were critical to Union support of the new state of West Virginia. The fort was evacuated on January 31, 1864, by Union Colonel Joseph Thoburn due to an impending attack by Maj. Gen. Jubal Early and nearly 5,000 Confederates. Early shelled the fort for some time before realizing that it had been evacuated. For the remainder of 1864 and 1865, action continued sporadically in the area, but Fort Mulligan was never again occupied as a garrison.