Fort Mercer was an earthen fort on the eastern shores of the Delaware River in New Jersey that was constructed by the Continental Army during the American Revolutionary War. The fort was built in 1777 by Polish engineer Thaddeus Kosciuszko under the command of George Washington. Along with Fort Mifflin on the Pennsylvania side of the D…Fort Mercer was an earthen fort on the eastern shores of the Delaware River in New Jersey that was constructed by the Continental Army during the American Revolutionary War. The fort was built in 1777 by Polish engineer Thaddeus Kosciuszko under the command of George Washington. Along with Fort Mifflin on the Pennsylvania side of the Delaware River to its west, Fort Mercer was designed to block the British advance on the revolutionary capital of Philadelphia during the Philadelphia campaign.