Fort Mercer is also the name of an unrelated location in the video game Red Dead Redemption.Fort Mercer was one of two forts constructed in 1777, on the Delaware River during the American Revolutionary War, by the Continental Army. Built by Polish engineer Thaddeus Kosciuszko under the command of George Washington, its purpose was to block the approach to Philadelphia, Pennsylvania: Fort Mifflin, on the Pennsylvania side, and Fort Mercer, on the New Jersey side.