 This is why General Oglethorpe later borrowed from Barnwell's idea for a fort to be built on St. Simons Island, where Fort Frederica, Oglethorpe's military base, was constructed in 1736.Additionally, he added forts and settlements near Skidaway Island, near the mouth of the Ogeechee River, near the headwaters of the Altamaha, on Cumberland Island, and on Amelia Island.This coastal defense system was instrumental in the eventual successful defense of Georgia under Spanish attack in the 1740s.