Three forts were part of a matrix of defenses to protect Philadelphia and control the Delaware River. Forts Mifflin and Mercer were just south of Philadelphia on both the Pennsylvania and New Jersey sides of the river. The third, Fort Billingsport, was a few miles to the south. General Thaddeus Kosciuszko was involved in the work on Fort Billingsport, his first engineering assignment authorized by the Continental Congress, but the full construction was never completed.
What is significant is that it was a strategic move led by two of the early American marine officers, Dennis Leary and William Barney, memorialized by a plaque on this site. Ultimately, both Forts Mifflin and Mercer fell, as did Philadelphia, but it did not mark the end of civilization. let alone the war, some were projecting.