Three gun batteries were built at Camp Hero, replacing most of the other heavy guns in the Harbor Defenses of Long Island Sound, which also included Fort H. G. Wright, Fort Michie, and Fort Terry. Two batteries of two 16-inch guns each were built, Batteries 112 and 113 (officially named Battery Dunn).[5] Another battery of two 6-inch guns was also built, Battery 216. All three batteries consisted mainly of a large concrete bunker covered with earth, containing ammunition magazines and fire control equipment.