Driving the George Washington Parkway north along the Potomac, you can almost miss the entrance to Fort Marcy Park. It’s not a well-known Civil War fortification, not being a sight of one of that war’s destructive battles, but it was one of the key components of the Union’s defense of the capital. (It’s also known as the place where White House Counsel Vince Foster’s body was discovered in 1993, but that’s not really relevant today.)
At the beginning of the Civil War, there was only one operational fort (Fort Washington, over in Maryland) to defend against Confederate encroachment. A huge effort was made to establish a defensive ring of forts around the capital, eventually resulting in a ring of eight enclosed forts and over 90 gun batteries by 1865. These preparations made DC one of the most heavily fortified cities in the world at that time.