The Demilune, so named because it (allegedly) resembles a portion of a circle, is really just a little spike of a triangle of a water battery that pokes out into the Delaware, added alongside Fort Mifflin in the 1870's. Whether the Demilune was ever fully armed is the subject of some thoughtful chin-stroking, but very few of the defensive structures built in the 1870's up and down the east coast of the US were completed.
Don't worry, this isn't a real cannon pitifully lying in such an undignified manner. It's fiberglass. Sure fooled the Royal Navy, though!