Fortifications

Fort Thayer (1861-1865) - A Union U.S. Civil War Fort established in 1861 in Northeast Washington DC. Named Fort Thayer after Colonel Sylvanius Thayer,  expert engineer known as the father of the United States Military Academy.  

Established in 1861 as a lunette with a perimeter of 180 yards and emplacements for eight guns. Fort Thayer was situated between Fort Saratoga and Fort Lincoln in the defensive ring around Washington DC. 

( Source - Fortwiki )