Moultrie saw Sullivan 's Island , a sandy spit of land at the entrance to Charleston Harbor extending north about 4 miles ( 6 @.@ 4 km ) long and a few hundred yards wide , as a place well suited to build a fort that could protect the entrance from intruding enemy warships . A large vessel sailing into Charleston first had to cross Charleston Bar , a series of submerged shoals lying about 8 miles ( 13 km ) southeast of the city , and then pass by the southern end of Sullivan 's Island as it entered the channel to the inner harbor . Later it would also have to pass the northern end of James Island , where Fort Johnson commanded the southeastern approach to the city . Moultrie and his 2nd South Carolina Regiment arrived on Sullivan 's Island in March 1776 , and began construction of a fortress to defend the island and the channel into Charleston Harbor . The construction moved slowly ; Captain Peter Horry of the Patriot naval detachment described the site as " an immense pen 500 feet long , and 16 feet wide , filled with sand to stop the shot " . The gun platforms were made of planks two inches thick and fastened with wooden spikes .