In 1777, George Washington's Colonial army was defeated at Brandywine by British forces. Led by General William Howe, 20,000 British troops moved into Philadelphia. He needed supplies that were on 200 British ships at the mouth of the Delaware River. Howe had to get his ships up the river before it froze in order to pursue Washington. What he didn't know was that there was a small contingent of troops in an unfinished British fort called Ft. Mifflin. Their objective was to delay the British ships.
The siege of Ft. Mifflin began in November of 1777. The British took over the fort after about a week. Before and during the battle, Washington arrived at Valley Forge and out of reach from British troops. The river began to freeze and Howe didn't get his supplies.
Fort Mifflin was used during the Civil War and the Korean War as a prison. It was closed in the 1960s and is now open for tours. Most of its' haunted past deals with the Civil War but there are other spirits that have been residents of the fort for much longer. A few hot spots include: The Officer's Quarters (the place where the Screaming Woman can be heard), Casemate 5 (location of the faceless man who is thought to be Billy Howe, the only man ever hung at the prison), and The blacksmith shop (Jacob the blacksmith always wanted the back door to be open and even after death, continues to open it).
More info: http://theshadowlands.net/famous/mifflin.htm
*Juliano, Dave (August 01, 2005). Fort Mifflin, Philadelphia, PA. Retrieved on August 13, 2005 on the Shadowlands website: http://theshadowlands.net.