 Atop a hill on the southern end of Seavey's Island stood an earthwork defense called Fort Sullivan, built in 1775 to supplement Fort William and Mary and Fort McClary in the protection of Portsmouth Harbor during the Revolution and, later, War of 1812.After 1815 it was abandoned, then reactivated from 1861 until 1865 during the American Civil War as an 11 gun, 8 inch Rodman naval battery.Another early battery was to the west on Henderson's Point.