At the end of the Civil War in 1865, the system of fortications (now known as Fort Circle Parks) which surrounded the capital city, Washington DC, were dismantled. The lumber and other materials were sold at auctions ad the land returned to pre-war owners. Fort marcy is perched 275.4 feet above the Potomac River on the south side of the road leading from Chain Bridge to Langley and McLean, Virginia. The perimeter of the fort is 338 feet. When completed the fort mounted 18 guns, a 10-inch mortar and two 24-pounder Coehorn mortars.

The hill on which the fort is located was known as Prospect Hill. Originally the fort was called Fort Baldy Smith, after General W.F. Smith, the troops of whose division began construction of the work. His division crossed Chain Bridge on the night of September 24, 1861 and immediately commenced construction of Fort Marcy and Fort Ethan Allen. The 79th New York Highlanders, the 141st Pennsylvania and the Iron Brigade also helped complete the work in the fall of 1862. A force of about 500 contrabands were also employed and the 152nd New York worked on the entrenchments, which incidentally are still in a very good state of preservation. The site of Fort Marcy is near the location where the famous but bloodless duel between Henry Clay and John Randolph was fought in 1826.

The fort was not entirely completed until the fall of 1862. It is a relatively undisturbed fort and was named in honor of a native of Massachusetts, the Honorable Randolph B. Marcy, a distinguished soldier and father-in-law and chief of state to General McClellan. Detachments of the 4th New York and 3rd Pennsylvania Heavy Artillery and the 130th Pennsylvania infantry were among the troops that performed garrison duty here during the war. An interesting incident of history is that the body of troops which afterwards became famous and the "Iron Brigade" was originated at this fort in the summer of 1861 and was composed of the 19th Indiana, the 2nd, 6th and 7th Wisconsin Regiments. The 24th Michigan was added to the brigade soon after the battle of South Mountain. These regulars remained together until the close of the war.

Source: http://www.nps.gov/gwmp/vapa/FtMarcy.htm