Having grown up in Philadelphia, I don't know how I missed this gem. Sitting right next to the airport, it's an easy thing to hit for a visit when you're coming or going or dropping someone off for their flight.
My first visit was on a day when I was flying out after visiting family and friends. I knew the Fort was by the airport, so I left early to have plenty of time to walk around. I was the only one there which made it that much more of a cool experience.
If you're interested more in the paranormal side of the Fort, check out the Ghost Hunters episode that featured this property - it's a good one!