Civil War Defenses of Washington
The Civil War Defenses of Washington were a group of Union Army fortifications that protected the federal capital city, Washington, D.C., from invasion by the Confederate States Army during the American Civil War. Some of these fortifications are part of a grouping of properties now managed by the National Park Service (NPS) and listed on the National Register of Historic Places.
Source: Wikipedia — Civil War Defenses of Washington (CC BY-SA 4.0)