Many visitors to the Charleston area are keenly aware of the famous fort that sits in the Charleston harbor. Fort Sumter played an important role in the American Civil War, as did Fort Moultrie over on Sullivans Island. But most people, even some locals, aren't exactly aware of a third fort used during that same war; a fort also located in the harbor and whose ruins now wither away under the constant laps of the sea and passage of time. Over the years, Castle Pinckney, located on Shutes Folly in the harbor, would be used for a number of purposes. Today, it's the only sea-bound abandoned prison in South Carolina.

Currently, after being named a U.S. National Monument in 1924 and then abolished as such (abandoned) in 1951, Castle Pinckney sits in a perpetual state of decay on top of decay. It's a prime example of what can happen when a national park is abolished in the U.S. (Note: Pinckney was a national monument, not a park.)

The fort was most notable for its rounded design, with artillery casemates positioned in the lower levels and pointing toward the opening of the harbor.

As it's seen today from the harbor, those openings for the cannons are no longer visible.

Either they have fallen below the waterline or were sealed off due to one of the uses for Castle Pinckney during the Civil War when it was converted into a prison.

At the start of the Civil War, Castle Pinckney and Fort Sumter were under the command of Union troops. Both would eventually fall under attacks from South Carolina Confederate forces.

After the first major land battle of the war, the First Battle of Bull Run which took place in Virginia, the Confederate victors transported 156 prisoners to Charleston where they were held at the City Jail until Castle Pinckney could be refitted as a prison. The gun casements on the ground level were all sealed off and converted to prison cells.

In the rare image below we see Confederate soldiers on the top level of Castle Pinckney overseeing the Union prisoners out in the courtyard.

Reportedly, the Union prisoners were allowed to roam the courtyard during the day and were only kept in their cells at night. While it wasn't exactly Alcatraz, conditions at the prison were crowded.

Castle Pinckney would serve as a prison twice during the conflict. Upon arrival in Charleston and while Castle Pinckney was being refitted as a prison, the prisoners were kept at Charleston City Jail. Six weeks after the Union soldiers were transferred to the newly refitted Castle Pinckney, the prison was deemed to be overcrowded. So the prisoners were transferred back to Charleston City Jail.

The second time Castle Pinckney would be used as a prison would come only six weeks after the Union prisoners had been transferred back to Charleston City Jail. After a major fire ravaged downtown Charleston in December 1861 causing damage to the jail, the prisoners were sent back to Castle Pinckney for approximately a week while the issues at the jail were sorted out.

Castle Pinckney's other uses throughout the years included serving as a federal storehouse as early as 1832 in anticipation of South Carolina seceding from the Union. In 1860 when South Carolina finally seceded the fort was manned by a lieutenant, a sergeant, four mechanics, and about 30 laborers. Only the lieutenant and sergeant were tasked with defending the fort.

By 1861, after the First Battle of Bull Run, the fort was refitted as a prison. In 1863, the Confederates refitted it as an artillery post. The photo above depicts the earthen mounds that were added inside the structure to the fort's already seven-foot-thick walls. Larger guns were added on the top level of the fort. In 1865, Castle Pinckney was returned to United States command.

Following the war, the fort was modernized in case it was needed during the Spanish-American War. It was not. By 1876 it had begun to be dismantled and a lighthouse was built on top. The lighthouse was in use until 1917.

In 1924, Calvin Coolidge declared Castle Pinckney a U.S. National Monument. However, access for the public to visit was never developed and in 1954, congress abolished the fort's national monument status, effectively abandoning Castle Pinckney to waste away to ruins in the sea.

In 2013, the Castle Pinckney Historic Preservation Society incorporated in South Carolina.

The mission of the non-profit is to preserve the fort and hopefully restore some of its lost features.

Keep reading to learn more about Castle Pinckney, the Castle Pinckney Historic Preservation Society, and yes, even learn more about its lesser known purpose as a now abandoned prison in South Carolina.

South Carolina had at least one other Confederate prison for Union soldiers and officers: Camp Sorghum, in Columbia.