 It occupies a commanding position at the head of Portsmouth Harbour.The fort is square, enclosing an area of 9 acres (36,000 m²) with outer walls 20 feet (6 m) high, 10 feet (3 m) thick, 210 yards (200 m) long and constructed of coursed flint bonded with limestone slabs.Square shaped forts became widely used during the 3rd century, being highly practical and defensible.