Delaware's position at the mouth of the Delaware Bay made the state vitally important as a military stronghold. Harbor defense was an important part of colonial history in Delaware, as well as in the Civil War, World War I, and World War II. Today, there are five abandoned forts in Delaware that remain a part of the First State landscape. Four of them are available to explore, but one remains a complete mystery.

1. Fort Christina

Swedish Settlers arrived in Delaware on March 29, 1638, aboard the ships Kalmar Nyckel and Fogel Grip, at a large stone outcropping which formed a wharf which is now known as The Rocks. The colonists ordered the construction of an earthwork fort around the Rocks, and Fort Christina was born. The Fort was rebuilt in the 1640s in response to the Dutch strengthening their nearby Fort Casmir, and tensions were high between Dutch, Swedish, and Native Americans in the area. In 1655, the Dutch launched a campaign to seize Fort Christina, and it was successful. Now renamed Fort Altena, the land became a Dutch possession, and the Swedes were driven out of North America. In 1938, to celebrate the 300th anniversary of the Swedish colonization of the area, the state of Delaware created a park which contained The Rocks and the site of the former fort. The dedication ceremony was attended by FDR and Swedish Royalty, who brought with them a monument topped with a replica of the Kalmar Nyckle. In 1961, the site was dedicated as a National Historic Landmark. In 2013, to celebrate the 375th anniversary of the landing, King Carl XVI Gustaf and Queen Silvia of Sweden, along with Joe Biden, reenacted the landing on a replica of the Kalmar Nyckel. In 2014, the site was combined with First State Historic Park, and is open to visitors from Memorial Day through Labor Day, 10AM to 4PM Tuesday through Saturday.

2. Fort Delaware

Fortification of Pea Patch Island began as early as 1812, when the US Army recognized the importance of the island in the Delaware Bay. The fortifications took many forms at first - and all failed. In 1848, construction began on a Fort that would not flop - what is currently known as Fort Delaware. During the American Civil War, Fort Delaware was held by the Union and used as a prison for Confederate prisoners of war, political prisoners, federal convicts, and privateer officers. Strengthening of the fort continued through the Spanish American War, and by 1900, the fort was working closely with Fort Mott in Pennsville, N.J. and Fort DuPont in Delaware City to create a three-point defense strategy.In 1947, the Fort was abandoned by the US Government, and the State of Delaware turned it into Fort Delaware State Park. Now, the Fort is rumored to be haunted - so much so that the TV series Ghost Hunters, Ghost Hunters Academy, and Most Haunted all featured the site. Fort Delaware is open to visitors Wednesdays through Sundays during the Summer, and special programs during the fall. You can find out more about these programs, including the Ghost Tours, on the Fort Delaware Website.

3. Fort DuPont

Fort DuPont was originally a small battery during the Civil War, but by the Spanish American War, it had become a full blown fort. In World War I, it was used as a defense fortification, as well as a training outpost for draftees, and for coastal deployment of artillery shipments. In World War II, the fort was an important mobilization station for deploying troops until the end of the war when all Delaware military activity was refocused and Fort Miles took the lead. After World War II, Fort DuPont served as a Health Center for Veterans. During WWII, about 300 buildings and structures lined the streets of Fort DuPont. By 2011, less than 80 historic buildings and structures remained.Present Day Fort DuPont is a state park, and restoration efforts are underway. The fort has a unique residential curatorship program which has allowed historical societies interested in it to restore old buildings at cost, with an agreement to not pay rent in using the building. You can visit the fort's website for more information about programs, or you can visit the fort. The park is open daily from 8 a.m. to sunset. Normal park office hours are Monday through Friday, 8 a.m. to 4 p.m. From April through October, the park office is also open weekends, 8 a.m. to 4 p.m.

4. Fort Miles

Fort Miles was built in 1941, and completed just days before the attack on Pearl Harbor. It was built to defend Delaware Bay and River and to protect domestic shipping from enemy fire from Germany surface fleets. The fort also operated a controlled naval minefield to prevent ships entering the Delaware River. At its peak, Fort Miles was home to over 2,200 soldiers, but it never saw major action during World War II. In May 1945, the soldiers at Fort Miles received the surrender of U-858, a German U-boat at the time of the German surrender to Allied forces in Europe.After the war, Fort Miles was abandoned and some sections have fallen into disrepair, but they're open to explore at Cape Henlopen State Park. We've written about Fort Miles before, if you're looking for a great guide to the entire site.

5. Fort Saulsbury

Fort Saulsbury was built in 1924 and remained dormant until World War II, when it was used as a heavy artillery battery as a harbor defense point, and a prisoner of war camp for German soldiers. In 1940, five fire control towers were built to support Fort Saulsbury, and one at Big Stone Beach still remains intact. With the construction of new batteries during World War II, almost all previous heavy weapons were scrapped by the end of the War. During World War II most of these new batteries were enclosed in heavy concrete casemates for protection against air attack, but Fort Saulsbury was one of the very few not casemated. Because of this, it is the best-preserved battery of its type in its original state.

In 1946, with the war over, Fort Saulsbury's guns were scrapped along with almost all other US coast artillery weapons. The fort was sold in 1948 to a private owner. As it is not open for visitors, the fate of Fort Saulsbury is a mystery.

Delaware's past as a strategic military location is not completely irrelevant in present day - after all, Dover Air Force Base is one of the most important Air Force Bases in the country. However, the days of harbor defense forts are behind us; advancements in technology have made forts like the ones above nearly useless. However, these abandoned forts in Delaware are critical to understanding the history of our state, and luckily most of those that are preserved are open to the public.