Fort Mifflin

The Quaker founders of Philadelphia famously built no fortifications around their new city, and the local government continued to oppose constructing a fort or raising a militia when the Spanish and French threatened the region in the 1740s. But in the 1770s the British ordered the fortification of the largest and most important city in their North American colonies, and the result is this remarkable piece of eighteenth-century military engineering, built on what was then called Mud Island, where the Schuylkill flows into the Delaware.

The British military engineer Captain John Montrésor (1736-1799) came to Philadelphia in April 1771 to lay out a fortress. Its star-shaped plan and raking masonry walls were designed to deflect the cannon fire of any who attempted to invade the city from the sea. Stonemason John Palmer was put in charge of construction, and by 1772 the three triangular “redans” facing across the river and one facing downstream had been built. Further work was stalled by engineering problems and stopped altogether in 1774, when the Pennsylvania Assembly, which was responsible for funding and which had previously pared down Montrésor’s proposal, declined to appropriate more money.

However, in 1775 Pennsylvanians’ security concerns grew and shifted from defense against France and Spain to Britian, and Benjamin Franklin’s Committee of Public Safety occupied the unfinished Mud Island Fort. By 1776 they had completed the circuit of its walls, although with earth and wood rather than stone.  At the same time Fort Mercer was built on the New Jersey side of the Delaware, and a ferocious, spikey barrier called a “chevaux de frise” (horse from Friesland) was stretched across the river between the two fortresses.

This defensive barrier deterred the British from their planned water-borne assault on Philadelphia in the summer of 1777, forcing them to march overland from the head of the Chesapeake Bay. But it did not prevent them from seizing the city on September 26.

Once in control, the British laid siege to the fortress that still compromised their ability to control the region. Montrésor was called upon to oversee the weeks-long assault on the fortifications that he had designed, firing on the fort from the west, where the walls were weakest.  A massive bombardment on November 10 turned the tide of battle, and the Americans withdrew on the night of November 15, but after setting the fort afire and without lowering its flag. The casualty rate was appalling: 250 of the 400 defenders were killed or wounded.
However, the British victory was pyrrhic. Washington had been given the chance to withdraw his troops to the safety of Valley Forge, the tempo of the British advance was slowed, and their supply lines were stretched. In June 1778 they left Philadelphia.

After independence was won, the Mud Island Fort—which in 1795 was named Fort Mifflin after General Thomas Mifflin (1744-1800)--became a vital component of the “First American System,” the new nation’s ring of coastal fortifications. In 1794 Pierre Charles L'Enfant (1754-1825), who had served as a military engineer in the Continental Army and authored the plan for Washington, DC, was commissioned to complete Montrésor’s fortress. Although he was replaced in this role by Lieutenant Colonel Stephen Rochefontaine (1755-1814) in 1798, the work that was completed in 1802 generally followed his design. This is the majority of the structure visible today.

L'Enfant replaced the hastily built wooden walls of 1776 with masonry—but largely brick, not the stone that Montrésor had used. It was also at this time that the three earliest surviving buildings within the fort were built, possibly to L’Enfant’s design. These are the austerely neoclassical “refuge,” a fortified place of retreat that was later designated the commandant’s house (1794-1796, cupola added 1835); the long, one-story barracks (ca. 1798), a functional structure with a porch running its entire length; and the simple blacksmith shop (before 1802, possibly 1778). The six cave-like casemates were also built in 1798-1801 to shelter soldiers from bombardment.

Later buildings include the officers’ quarters (with an elegant cast iron railing on the second-floor porch, 1814), the guardhouse (later arsenal, 1815-16), the hospital and mess hall (located outside the walls, ca. 1820), the artillery shed (1837), and the quartermaster’s storehouse (1843).

The construction in 1815-1824 of Fort Delaware on Pea Patch Island (300 miles downstream) made Fort Mifflin militarily obsolete. It served as a prison camp during the Civil War, and the Army Corps of Engineers established a base here (and still occupies an adjacent piece of property). After a campaign to recognize the fort’s significance and protect it, ownership was transferred to the city in 1956, and it was opened to the public in 1968.