Built in 1757 by the British during the French and Indian War to protect the frontier settlers, ironically years later it was used as a prison for British soldiers during the American Revolutionary War, and during the Civil War Union troops were stationed here to guard the Chesapeake and Ohio (C&O) Canal from Confederate Soldiers. In the 1930s, the fort had been neglected for years and was in ruins, however the Civilian Conservation Corp (CCC) restored it using the original plans.
Today, the site is part of Fort Frederick State park, where one can tour the fort, visit the CCC museum, camp, access the C&O Canal.
History taken from wikipedia:
French and Indian War
Fort Frederick was built in 1756-57 by the colony of Maryland. During the French and Indian War in 1756, a £6000 appropriation was authorized by the Maryland Legislature at the request of Governor Horatio Sharpe to build a fortification on the frontier. The fort, named after Frederick Calvert, 6th Baron Baltimore, was completed the following year. The design of the fort conforms to the style developed early in the 18th century by Sebastien de Vauban, a French military engineer who is considered the father of modern fortification.
The large stone fort was designed primarily as a place of refuge for area settlers. Between 1757 and 1758, small raids by Native people in nearby settlements caused settlers in the surrounding countryside to flee eastward. At the same time men of the 60th Regiment of Foot and local militia soldiers garrisoned the fort. Ranging parties were sent from the fort to patrol the area and to deter if not prevent raids by Native Americans.
The fort was not designed to resist artillery, as it was correctly assumed that the French would not be able to transport artillery to the remote location from the west. The fort served its purpose in 1763 during Pontiac's Rebellion; however, the fort was never directly attacked.
American Revolutionary War
The fort was used as a prisoner of war camp from 1777 to 1783. As many as 1,000 captured British and German soldiers were incarcerated there after the Battles of Saratoga (1777) and Yorktown (1781).
American Civil War
Fort Frederick was sold at auction in 1791 and lay abandoned until the American Civil War. The fort was garrisoned at the outbreak of war and was used as a gun emplacement to protect the Chesapeake and Ohio Canal and the Baltimore and Ohio Railroad, which paralleled the canal. The 1st Maryland Infantry (US) occupied the area in December 1861 and Company H fought in a skirmish at the fort against Confederate raiders on Christmas Day, 1861. The regiment left in February 1862. In October 1862, a picket from the 12th Illinois Cavalry briefly occupied the area. The military usefulness of the fort ended by 1862.
20th Century
In 1922, the property was acquired by the State of Maryland for use as Maryland's first state park. The walls had deteriorated but were standing up to 8 feet (2.4 m) in places. Archaeological investigations and the discovery of the original plans allowed a complete reconstruction. Much of the restoration work of the 1930s was done by the Civilian Conservation Corps.