You might not know it, but Washington, DC was once one of the most heavily fortified cities in the world. To the casual observer, the only evidence of the 37 mile ring of defenses that surrounded the city during the Civil War is the Fort Totten metro stop. But go exploring and you’ll find important remnants of American history, and some pretty neat parks, taboot.

By the end of the Civil War, Washington was defended by 68 earthwork forts, which were made from thick, reinforced dirt walls. “Dirt is a very inexpensive resource,” as the National Park Service puts it.

The defenses were well needed. In the summer of 1864, Ulysses S. Grant had pursued the Confederate army south to Richmond, Virginia. Robert E. Lee sent General Jubal Early to take advantage of the relative lack of soldiers in Washington and attack the capital. Grant scrambled to send reinforcements, who made it to Washington just as Early and his troops were preparing to attack Fort Stevens, at what is now 13th and Rittenhouse St, NW.

President Abraham Lincoln and his wife Mary Todd rode up from the White House to observe the skirmish (watching battles was a thing in the Civil War). As Lincoln stood on top of the fort’s parapet, Confederate soldiers—who might have recognized the 6’4” president—trained their weapons on him. This was the only time a sitting president has ever been fired on during wartime. According to one possibly apocryphal story, Oliver Wendell Holmes, Jr. got the president to take cover by yelling, “Get down, you damn fool!” Holmes of course went on to serve on the Supreme Court.

While many of the forts were demolished as the city grew, several of them, and the land that connected them, fell into the hands of NPS. They now make up Fort Circle Park—officially, the Civil War defenses of Washington. If you squint, you can make out a ring of green wrapping around the city.

Some forts are still standing today, in one form or another. Fort Stevens has been partially reconstructed, and the nearby Battleground National Cemetery is well maintained. Fort Totten, which lends its name to the metro stop and surrounding neighborhood, sits at the top of an easily climbed hill. Fort DeRussy is tucked away off the path that runs along Military Rd, NW. South of the Anacostia, Fort Dupont, Fort Chaplin, and Fort Mahan are in good condition, and are connected by a popular hiking and mountain biking trail.

An ambitious bike rider could hit all the forts in one day. But a better plan? Pick one or two to explore.