Fort Mifflin was built on a low marshy island near the mouth of the Schuylkill. Its very situation, surrounded as it was by mud and water, made it impregnable to any land attack. While the fort itself was a fairly strong earthwork, laid out upon approved principles of engineering, its outer works of defence added greatly to its strength. In the main channels of the river were sunk heavy, sharp-pointed _chevaux de frise_, or submarine palisades, with sharp points extending just above the surface of the water. In addition to this obstacle, the enemy advancing by water upon the fort would have to meet the American flotilla, which, though composed of small craft only, was large enough to prove very annoying to an enemy. In this flotilla were thirteen galleys, one carrying a thirty-two pounder, and the rest with varying weight of ordnance; twenty-six half-galleys, each carrying a four-pounder; two xebecs, each with two twenty-four-pounders in the bow, two eighteen-pounders in the stern, and four nine-pounders in the waist; two floating batteries, fourteen fire-ships, one schooner-galley, one brig-galley, one provincial ship, and the brig "Andrea Doria." It was no small naval force that the British had to overcome before attacking the mud ramparts and bastions of Fort Mifflin. Against this armament the British brought a number of vessels, with the "Augusta," sixty-four, in the lead. The battle was begun late in the afternoon of the 22d of October, 1777. The attack of the Hessians upon the American fortifications at Red Bank, and the opening of the action between the British and American fleets, were simultaneous. The Hessians were beaten back with heavy loss, some of the American vessels opening fire upon them from the river. The naval battle lasted but a short time that night, owing to the darkness. When the battle ended for the night, the "Augusta," and the "Merlin," sloop-of-war, were left hard and fast aground.