At the heart of America's Third Fortification System were large, all-masonry forts with most of their guns protected by casemates - masonry enclosures with small openings for firing. By the middle of the nineteenth century, most large seaports and major river mouths were protected by one or more forts. Forts Jackson and Pulaski guarded the Savannah River below the city of Savannah, while the guns of Fort Clinch on Amelia Island covered the St. Marys River and Cumberland Sound.
The perfection of rifled artillery with increased accuracy and impact made masonry forts obsolete. Fort Pulaski bears the marks of a thirty-hour Union bombardment in 1862 that established forever the vulnerability of brick and mortar to rifled guns. As a result, new forts built during the Civil War, like Fort McAllister on the Ogeechee River in Georgia, relied again on earthen walls. Coastal installations of the later nineteenth and early twentieth centuries were small batteries, usually of two guns, protected by concrete and earth parapet walls. Examples of these can be found at Fort Screven on Tybee Island near Savannah.