Guarding Philadelphia's southern shores since 1771, Fort Mifflin remained an active U.S. military stronghold from the Revolutionary War until it was decommissioned in 1959. The fort was the site of the Siege of Mud Island Fort (also known as the Siege of Fort Mifflin) in 1775, which was a battle of immense strategic importance for both the Royal British Naval Fleet and the American colonial soldiers. The fort held out for nearly two months before it yielded its position to the invaders. During the U.S. Civil War, the installation was used to contain Confederate prisoners of war. The fort's contribution towards the protection of the city was officially recognized on the 29th of August 1970, when it was designated as a National Historic Landmark. Today, it is not unusual to spot period uniform and weapons demonstrations regularly on its hallowed grounds.