The Castle Gatehouse at Georgetown Reservoir was constructed between 1899 and 1902 as part of improvement and modernization efforts for the Washington Aqueduct. The gatehouse was designed to pump water from the Georgetown Reservoir into a four-mile long tunnel toward the McMillan Reservoir — one of several major efforts put in place to address the lack of filtration in the original Aqueduct system.  The gatehouse’s design is a nod to the Corps Castle, which was formally adopted by the U.S. Army in 1902 as the insignia of the Corps of Engineers, said to represent strength, pride, heraldry, prestige, and honor.  The castle gatehouse was designated as a National Historic Landmark in 1973 and was added to the National Register of Historic Places in 1975.