Long before it became a hipster beach destination, Fort Tilden was a US Army Artillery Post constructed in 1917 to protect New York Harbor during WWI. It was named in honor of former New York Governor Samuel J. Tilden. In 1941, when it became clear that the US was going to enter the war, as many as 1,000 men were stationed at Fort Tilden. During the Cold War in the mid-1950s, Fort Tilden was used as a Nike Missile installation site. In 1974, the Army transferred Fort Tilden to the the National Park Service’s Gateway Recreation Area. Today, Fort Tilden is not only a summer haven, but also the site of public art projects like MoMA's recent "Rockaway!" installation.

Located on a hilltop in Central Park’s North Woods, BlockHouse No. 1 claims the distinction of being the park’s oldest structure. It was built as part of a series of fortifications in the summer of 1814 to defend against the British after they attacked Stonington, Connecticut. However, the Blockhouse (and the rest of New York’s forts built during this time) were never used in combat, as the Treaty of Ghent was signed a few months later, ending the War of 1812. Today, the the blockhouse is “empty, roofless, and securely locked.”

Fort Totten was built in 1862 to help defend New York Harbor during the Civil War. After the war, it served various other military functions, most recently as an Army Reserve post during the 1970s. In 1987, the US Department of Defense gifted 10 acres of the peninsula to the NYC Parks Department, and that land served as the basis of the park that would continue to grow into the 2000s. The park today is still home to a late 19th-century Gothic Revival castle, which originally used as an Officers’ Mess Hall and Club for the U.S. Army Corps of Engineers School of Application. Today, it houses the Bayside Historical Society.

Located on the western point of Governors Island, Castle Williams was designed designed by Benjamin Franklin’s cousin Lt. Col John Williams, the chief engineer of the US Army Corps of Engineers. Completed in 1811, it was also a part of the Second American System. Like Fort Jay, Castle Williams was used as a Confederate prison, though it was overcrowded and disease ran rampant. In 1895, the fort was designated as one of 10 military prisons in the US, and became known as “Alcatraz East.” The Coast Guard took over the building in the late 1960s, and used the fort until the Guard left Governors Island in the mid-1990s.

Battery Park’s Castle Clinton was also constructed to defend New York Harbor from the British. Originally named Southwest Battery, the fort was completed in 1811, though its 28 cannons were never fired during the War of 1812. The fort was renamed Castle Clinton in 1817 in honor of Dewitt Clinton, the mayor and late governor of New York. From 1823-1854, the fort was used as Castle Garden, an opera house and theater. In 1855, the fort became America’s first immigrant receiving center, welcoming 8.5 million immigrants until Ellis Island opened in 1892. But Castle Clinton was transformed yet again in 1896, as the New York City Aquarium (which moved to Coney Island in 1941). The building was saved from demolition in 1946, despite NYC Parks Commissioner Robert Moses calling it a “bombed-out gas tank,” and Castle Clinton National Monument opened in 1975. Today, the site houses the ticket office for the Statue of Liberty.

Fort Tryon honors two Revolutionary War soldiers who fought in the November 1776 Battle of Fort Washington:General Sir William Tryon, who led the British to victory, and Margaret Corbin (for whom the Park’s Drive and entrance are named), the first woman to ever receive a military pension for her service to the Continental Army. After Corbin’s husband John was killed during the battle, she commandeered his cannon, and was the last to stop firing despite being severely wounded. In 1917, John D. Rockefeller, Jr., began purchasing some of the land in order to build a park overlooking the Hudson River and Palisades. He hired Frederick Olmsted Jr., the son of the designer of Central and Prospect Parks, to develop a plan; the park opened in 1935, and the Cloisters opened in 1938, after Rockefeller purchased a collection of medieval art. In addition to hosting an annual Medieval Festival, the park today claims the distinction of being home to Manhattan’s largest dog run.

Staten Island's Fort Wadsworth is one of the oldest military sites in America, having been initially fortified by the British in 1779. During the War of 1812, Americans used the fort to guard the Narrows (between Staten Island and Brooklyn). The first permanent troops were stationed at the Fort during the Civil War. After WWI, the Fort was used as an army post, and it became the site of an Army Chaplain School from 1974-1979. After the army left the site, the fort became the headquarters of the Naval Station New York, and in 1994 it became the part of the Gateway National Recreation Area. The Coast Guard still uses some of the Fort’s buildings and housing.

NYC's only remaining active-duty military post is Fort Hamilton, located at the southern tip of Brooklyn near Bay Ridge. The Fort was constructed in from 1825-1831 as part of the Third System Fortifications, and named in honor of the now überpopular Alexander Hamilton. During WWI and WWII, the Fort was used as a staging area for American troops who were deployed overseas. Fast forward a few decades, and members of the military at Fort Hamilton played an important role in relief efforts after the September 11 attacks and Hurricane Sandy in 2012. After Sandy, the Fort became a Base Support Installation, hosting organizations including the FBI and FEMA, who helped with relief efforts.

Fort Wood was constructed on today’s Liberty Island in 1807 in order to defend the city from enemy assault. The 11-point fort was named after Eleazer Derby Wood, an officer who died in the War of 1812 during the Battle of Lake Erie. But now, the structure serves a rather different purpose: It was repurposed as the star-shaped base of the Statue of Liberty, and visitors to the tourist spot can still see it to this day.