One of the most effective defenses Fort Mifflin employed against the British navy are called Chevaux-de-Frise. Designed by Benjamin Franklin (1706-1790), they are best described as timber cribs. These large handmade boxes are constructed by many crews between 1775 and 1777. The cribs themselves are up to 40 feet in length, up to 25 feet in width, and up to 15 feet in height. To keep them firmly in place at the bottom of the Delaware Riverbed, each crib is filled with tons of stone. Rising from the crib at a 45-degree angle are timber logs up to 35 feet in length, fitted with forged iron tips that sit just below the water line. When filled, each Chevaux-de-Frise weighs up to 40 tons.

Instead of arranging them tightly together, they are placed in staggered rows with narrow channels for the Americans to navigate through. As the large British ships pass over them, the iron tips pierce the hull of the ship, leaving it severely damaged.