The American Civil War saw a wide array of new forts spring up across America, while many older ones were called into service. There's Fort Sumter, where it all started, and Fort Gaines, which was the site of a key battle. Others had more low-key but no less vital roles to play in the conflict, and they're all fascinating examples of historic military architecture.

Many of these structures are now open to the public. Click through this gallery to see the amazing forts that shaped the Civil War….

Fort Gaines was a crucial site for the South during the Civil War, and is famous for its part in the Battle of Mobile Bay in 1864. In order to close the bay to blockade running, Union troops laid siege to the fort while a fleet of 18 ships under the command of Admiral Farragut entered the bay. They received heavy fire from Confederate forces in Fort Gaines and Fort Morgan, at which point Farragut shouted his now-famous order “Damn the torpedoes! Full speed ahead!” After passing the forts he forced the Confederate navy to surrender, effectively closing the bay.

Fort Gaines' sister fort, the larger Fort Morgan, was located on the opposite side of the channel at the entrance to Mobile Bay, and was the last big holdout in the battle. It eventually surrendered unconditionally on 23 August 1864 after two weeks of heavy artillery fire, shutting down the port and ending the fight. Both Fort Gaines and Fort Morgan are now preserved as historic sites and are open to visitors. 

Fort Jay is considered one of the finest examples of classic star-shaped fort design anywhere in the US. First built in 1776, it was reconstructed several times and renamed Fort Columbus (it reverted to Fort Jay in 1904) before the Civil War began. During the Civil War it served as a Union prison for Confederate prisoners, housing officers in relatively comfortable conditions. Governors Island’s military role ended in 1995 and in 1996 it became a New York Historic District. The fort has been open to the public since 2005.

Built by the British in 1771, Fort Mifflin was badly damaged in the Revolutionary War, but returned to service during the Civil War and served as a military prison. A breakdown in prisoner exchanges in 1863 meant finding places to hold thousands of extra prisoners, while Fort Mifflin also held civilians and Union soldiers guilty of violations of military conduct. Now a National Historic Landmark, the fort hosts guided tours, living history programmes and a wide variety of other events including historic reenactments. 

Fort Sumter is, of course, where it all began. It was one of several forts surrounding Charleston Harbor that were supposed to work together, but instead became opponents. When South Carolina seceded from the Union in December 1860, the Federal garrison abandoned Fort Moultrie for the stronger Fort Sumter. A few months later Confederate troops shelled it into submission – plunging the nation into civil war.

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While Fort Sumter, which suffered an 18-month bombardment by Union troops later in the war, was left barely more than a pile of rubble by the war's end in 1865, Fort Moultrie lay hidden under a band of sand that protected its walls from the shells. The then-incomplete fort had already seen battle in the Revolutionary War, repelling nine British warships in 1776. Today, the remains of Fort Sumter and Fort Moultrie make up a National Historical Park, and can be visited on ferries via Fort Sumter Tours.

Completed in 1777, Connecticut's Fort Trumbull served as a Union fort during the Civil War. It was primarily used as an administrative centre for inducting and training recruits, and was also the headquarters of the 14th Infantry Regiment. It didn't see much real action but has achieved literary fame thanks to Mark Twain, who used the fort as a setting for his short thriller, A Curious Experience. Today the fort serves as a public park and tourist attraction.

Astonishingly Fort Washington, completed in 1809, spent many years as the only permanent fortification defending the nation's capital. Indeed, prior to the Civil War it was DC’s only defence, save for a few guns at the Washington Arsenal. The war saw a circle of temporary forts built around the city with Fort Washington garrisoned as its outer defence, and companies of the First and Fourth Artillery passed through as the war wore on. Set on the Potomac River, it's one of the few remaining seacoast forts to survive in its original design, and today is open to the public.

Located on Pea Patch Island in the middle of the Delaware River, Fort Delaware was initially constructed around 1859 to protect Wilmington and Philadelphia from enemy attack. The fort was state-of-the-art by the standards of the time, and during the Civil War it became a Union prison camp, holding a maximum of 12,595 Confederate prisoners of war. Fort Delaware became a state park in 1951 and today is a popular tourist attraction, with costumed reenactors transporting visitors back to the Civil War era.

Old Fort Jackson is Georgia’s oldest standing brick fortification. Construction began in 1808, and it was expanded in the 1840s and 1850s. For most of the Civil War Confederate forces used it as their headquarters for defending the Savannah River, but it was then garrisoned by Union troops after the surrender of Savannah in 1864. Today it is a National Historic Landmark offering daily history-themed programmes that are particularly aimed at families.

Fort Pulaski is a massive, five-sided structure that was long considered virtually indestructible. However, it proved inadequate against the new rifled cannons introduced in the Civil War era. In 1862 Union forces amassed 20 cannons and 14 mortars to bombard the Confederate-held fort, and within two days two 30-foot (9m) holes had opened in its southeast face. Union forces took the fort that same day, for the loss of just one soldier. The siege is considered a turning point in US military history. Today the fort is a National Monument open to visitors.

Fort Independence, formerly known as Castle William, sits on Castle Island in Boston Harbor. There has been a fort here since the 1630s, although the original was set on fire by retreating British forces during the Revolutionary War. It was then rebuilt as a star-shaped fort and renamed Fort Independence in 1799. During the Civil War the fort served as an armoury, with the Union Army's Fourth Battalion of Massachusetts Infantry stationed here. Today it is open to the public between Memorial Day and Columbus Day.

Dedicated in 1847, Fort Warren’s design was practically obsolete by the time it was completed in 1861, just as the Civil War broke out. During the war the fort served as a training ground and as protection for Boston Harbor, its ramparts boasting an impressive 97 guns. It was also used as a military prison, and had an unusually positive reputation for humane treatment of prisoners. The fort was decommissioned in 1947 and today is open to the public, with park rangers and volunteers offering guided tours.

The Union-held Fort Schuyler in New York was garrisoned by engineers in January 1861, who occupied it until 1865 when it was turned over to artillerymen. As with so many forts it was used as a prison during the war – holding up to 500 Confederate prisoners in total – and housed the 2,000-bed McDougall Hospital until it was destroyed by fire. Today the fort is home to the State University of New York Maritime College and the Maritime Industry Museum.

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Completed in 1834 on Santa Rosa Island near Pensacola, Fort Pickens was one of only four southern forts to remain in Union hands throughout the Civil War. An initial truce saw the North agree not to reinforce the fort and the South agree not to attack it, but several exchanges of fire followed, including an unsuccessful assault by Confederate troops. The pressure eased in 1862, when most Confederate forces in the area were redeployed further north. The fort was in use until 1947 and today it is open to the public year-round.

Completed in 1844, and built on the ruins of earlier forts constructed by the Spanish, French and British, Fort Barrancas provided a vital line of defence for the Pensacola Navy Yard. The only time it ever saw combat was during the Civil War, when it was occupied by Confederate troops and exchanged fire with nearby Fort Pickens during the winter of 1861-62. Abandoned by the South in May 1862, it saw no further action as the war played out and is today part of the Gulf Islands National Seashore.

The southernmost Civil War-era fortress in America, Fort Zachary Taylor remained a Union stronghold throughout the conflict, providing a vital deterrent to Confederate ships and 'blockade runners' carrying cargo for the South, and also serving as a military prison. The fort later played a defensive role in the Spanish-American War, and remained in military use until 1947. Declared a National Historic Landmark in 1973, it has been open to the public since 1985 and boasts the largest collection of Civil War cannons in the United States.

The largest brick masonry structure in the Americas, Fort Jefferson was under construction for almost 30 years (1846-75), and was never quite completed or fully armed. Located among the islets and shoals of the Dry Tortugas in the Gulf of Mexico, it provided a key defensive position on one of the world’s busiest shipping lanes, allowing the Union navy to blockade Southern shipping during the Civil War. Abandoned by the army in 1874, it’s now a major tourist attraction, easily accessible by boat or seaplane from Key West.

Although only two-thirds completed at the outbreak of the Civil War, lacking cannon and with lower-than-planned walls, Fort Clinch served as a coastal base for Confederate troops from April 1861 to March 1862, when General Robert E Lee gave the order to evacuate. Union troops took the fort that same month. The fortress fell into disrepair after the Spanish-American War but a restoration programme began in 1936, and it became the centrepiece of one of Florida’s first state parks. Today it's open to the public, and hosts regular Civil War reenactments.

Fort Massachusetts was not yet complete when Mississippi seceded from the Union in January 1861 and militia forces took control of Ship Island. They abandoned it in September, setting fire to the semi-completed fort as they did so. Union forces resumed construction, and prisoners on the island were guarded by the 2nd Regiment of the Louisiana Native Guards, which would become the first Black unit to see combat on the Gulf Coast in 1863. Today the island is open to the public from March to October.

Fort Rodman was intended to serve as a link in a coastal chain of forts between Portland in Maine and Long Island, New York. War broke out before the fort was finished, so the local mayor decided to construct an earthworks fort next to it as a placeholder. This fort was armed and operational throughout the Civil War, while Fort Rodman was never completed even after the war's end. The partially completed fort used to be open to the public but safety concerns mean that it's now been closed for more than a decade. 

On the day that Virginia officially joined the Confederacy, three enslaved people fled to Fort Monroe to seek refuge after learning that their master intended to send them to North Carolina to support secession forces. Their master demanded their return under the Fugitive Slave Act, but Union Major General Benjamin F Butler claimed that the Constitution and Fugitive Slave Act did not apply in another country, which Virginia now claimed to be. Butler’s declaration – known as 'the Contraband Decision' – enabled thousands of enslaved people to seek refuge behind Union lines. Now a National Monument, Fort Monroe is open to the public. 

Built in 1862 on the banks of the Kennebec River, Fort Popham’s main purpose was to protect Maine’s capital, Augusta, from Confederate attack. Constructed almost entirely of granite blocks, the imposing fort boasts an unusual crescent shape with a circumference of 500 feet (150m) and walls 30 feet (9m) high. Its formidable defences included 36 cannons, each of which fired shots weighing 220kg (480lbs). The fort was rendered obsolete by the construction of nearby Fort Baldwin in 1905, but is now a popular tourist destination.

The largest inland stone fortification constructed during the Civil War, Fort Negley was built after Union troops captured Nashville in February 1862. The fort, with its complex star-shaped design, was built mostly by enslaved people and free Black conscripts – roughly 25% of whom died during construction from disease, injury or exposure. The fort – particularly its artillery – played a key role in the Battle of Nashville in 1864, but fell into disrepair after the war. Following a lengthy restoration project, it finally reopened to the public in 2004. 

Fort McAllister, a Confederate earthwork fortress built in 1861, played a vital role in defending Savannah during the Union blockade of the Georgia coast. It successfully repelled seven attacks by Union ironclad warships, aided by its unusual earthen construction. It held out until December 1964, when General William T Sherman easily took the fort from the landward side at the end of his 'March to the Sea'. Purchased by Henry Ford in the 1930s, the fort was restored to its former glory and is now part of Fort McAllister State Park.

With origins dating back all the way to 1672, Fort Castillo de San Marcos is the oldest masonry fort in the continental United States. Originally built by the Spanish, its walls are constructed from coquina, an incredibly durable limestone composed of broken shells. Renamed Fort Marion in the 1820s, it followed the usual pattern for Florida forts during the Civil War – claimed by the Confederacy, but surrendered to the Union in 1862. In 1933 the fort was handed over to the National Park Service, and its original Spanish name was restored in 1942.

Originally known as the Fort at Willet’s Point, Fort Totten was built during the Civil War in 1862. Along with Fort Schuyler on the opposite bank of the East River, it was built to defend the Eastern approach to New York Harbor. Its initial design was drawn up by Robert E Lee, but its defensive purpose was short-lived as masonry forts were considered almost obsolete by the war's end. After the war it served a variety of military functions, and since the 1980s has formed part of a public park.

Fort McHenry is probably best-known for the valiant role it played in the Battle of Baltimore in 1814, which inspired Francis Scott Key to write the words that would become The Star-Spangled Banner. During the Civil War it served as a prison for the large number of Maryland citizens who expressed pro-secessionist sentiments, including Francis Key Howard, the grandson of Scott Key. Designated a National Monument and Historic Shrine in 1939, it is now a popular spot with tourists eager to see 'the Birthplace of the National Anthem'. 

Now check out these extraordinary black-and-white photographs from the Civil War