Fortress in south Baltimore, on a piece of land called Whetstone Point. The site was chosen to defend Baltimore harbor without being too close to the bulk of the city.
Though built during the American Revolution, it saw no action until the War of 1812, when it sustained a 25-hour bombardment from British ships in the Patapsco River. The defenders of the fort held the day, and Baltimore was saved. A man named Francis Scott Key happened to be around to see the American victory, and was moved to pen a little ditty about it.
During the American Civil War, the fort was occupied by Union troops and its guns trained on the city, which was full of Confederate sympathizers and kept under martial law. It was also used as a prison where almost 7000 Confederate prisoners of war were detained.
In 1917 the nation's largest military hospital was built at the fort, to care for wounded vetrans of World War I. The hospital gradually fell into disuse and was torn down in 1925.
Since 1925 the fort has been a national park of one kind or another; in 1939 it was designated the nation's only Historic Shrine.
Some information condensed from http://www.bcpl.net/~etowner/history.html