Pea Patch Island
I was nearly right:
"The War of 1812, which included the burning of Washington DC and bombardment of Baltimore, propelled America into a frenzy of coastal defense, and the first fortification of Pea Patch Island took place in 1813. A plan was adopted by Congress in 1816 to build 200 coastal forts, and about forty of them were actually completed by the time of the Civil War. In essence, these forts were huge walls of bricks with a concrete outer shell, holding a couple dozen very large cannons and a parade ground.
The river defenses of Philadelphia were provided by three forts, Fort DuPont at the mouth of the old Delaware-Chesapeake canal, Fort Delaware on the island, and Fort Mott on the New Jersey side. You can now take a ferry between all three, between April and September; it’s a pleasant afternoon excursion. Not so many years ago, you had to go into an ominous little taproom in Delaware City and ask in a loud voice if someone wanted to take you to Pea Patch in a fishing boat. The scene was reminiscent of old movies about derelicts hanging out in Key West, complete with George Raft and Earnest Hemingway, but now the National Park Service has given it the characteristic NPS sprucing up, with pamphlets and rest rooms.
The place never had any serious military activity except when it was used to house Confederate prisoners after the Battle of Gettysburg. Over 12,000 prisoners were brought there, and there were about 3,000 deaths among them. Historians have compared the treatment of Confederate prisoners with the treatment of Union prisoners at Andersonville, Georgia, but it’s hard to say which place was worse. There are certain diseases of poor sanitation, like typhoid, cholera, amoebic and bacillary dysentery, and hepatitis, which decimate all concentration camps at all times. And adding to them the mosquito-borne diseases of both Delaware and Georgia at the time, you don’t really need to assert that there was a lot of prisoner mistreatment to account for the morbidity and mortality. Undoubtedly there was some of that."