During the American War for Independence, Fort Frederick was revitalized for military purposes when the Continental Congress turned it into a prison camp for captured British soldiers. The fort became extremely overcrowded, local militia usually guarded the prisoners, and escape attempts were frequent. After the war, Maryland found no further use for the fort and sold it with 99.5 acres at public auction in the 1790's. Throughout most of the 19th Century, Nathan Williams, an African American who purchased the property in 1860, farmed Fort Frederick with his family, acquired additional land, and constructed several dwellings and a barn in the northwest bastion; during the five years of Williams family occupation, one of Washington County's first schools for African Americans was also established on the property. The fort and surrounding grounds continued in agricultural use until the outbreak of the Civil War in 1861, by which time the barracks had disappeared and the fort was little more than a large stonewall. During the winter of 1861-62, a company of Union soldiers from the 1st Maryland Regiment garrisoned the fort, and on Christmas Day, 1861, they skirmished with Confederate raiders who had crossed the Potomac River. After the main theater of action shifted elsewhere in 1862, the fort was abandoned again, ending its military use. By the late 19th Century, public interest in preserving Fort Frederick for its historic value had increased, and it became a popular site for patriotic celebrations. In 1922, Maryland acquired the fort and 189.5 acres, and during the 1930s the Civilian Conservation Corps established a camp there, conducted archeological excavations, reconstructed the fort walls, and capped the original building foundations at ground level, helping preserve Fort Frederick and develop it as a State Park.
