Only through sheer determination did I find this (I'm not very smart).
Thanks for the "X marks the spot."
Fort Mifflin, originally called Fort Island Battery and also known as Mud Island Fort, was commissioned in 1771 and sits on Mud or Deep Water Island on the Delaware River in Philadelphia, Pennsylvania. During the American Revolutionary War, the fort was a centerpiece of the British conquest of Philadelphia. The name "Fort Mifflin" became official in 1795. The fort was rebuilt at the start of 1794 during the presidency of John Adams to a design by Pierre L'Enfant, and added to in the 19th century.