Delaware River, guarding against General Howe and Admiral Reynolds' advance naval fleet on the Delaware. Defenders of Philadelphia assembled chevaux de frise obstacles, placed in tiers spanning the width of the Delaware between Forts Mercer and Mifflin. These defenses comprised wooden-framed "boxes", 30 feet square, constructed of huge timbers and lined with pine planks. Defenders lowered these frames onto the riverbed and filled each with 20 to 40 tons of stone to anchor it in place. They placed two or three large timbers tipped with iron spikes into each frame, set underwater and facing obliquely downstream. They then chained the boxes together to maintain continuity. The chevaux de frise presented a formidable obstacle that could impale unwitting ships. The system's design included gaps to allow passage of friendly shipping. Only a select few patriot navigators knew the locations of safe passage through this barrier. Soldiers at Forts Mercer and Mifflin could fire at anyone attempting to dismantle these obstacles. Similar obstacles were built downriver at Fort Billingsport, NJ, but that area fell to the British on October 2, 1777. After the defeat of Washington at the Battle of Brandywine, the British took control of Philadelphia in September 1777 during their Philadelphia Campaign. The British forces then laid siege to Fort Mifflin and Fort Mercer in early October 1777, unsuccessfully attacking the latter by land and river in the Battle of Red Bank on October 22. The British Army intended the siege to open up its supply line via the Delaware River. Captain John Montresor, earlier designer and constructor of Fort Mifflin, planned and built the siege works used against Fort Mifflin. He then led the siege and destroyed much of the fort. During the siege, four hundred American soldiers held off more than two thousand British troops and 250 ships until 10 November 1777, when the British intensified their assault, launching an incessant barrage of cannonballs into the fort. Among those stationed at the fort was private Joseph Plumb Martin, who later wrote an account of the battle. Defending the riverway Commodore John Hazelwood with a sizable fleet of galleys, sloops, and fire-vessels launched several raids on British positions on shore, constantly harassing British river operations while patrolling the waters around the fort. On 15 November 1777 the American troops evacuated the fort. Their stand effectively denied the British Navy free use of the Delaware River and allowed the successful