The walls are constructed of tabby, which is made largely from oyster shells.

Destined to defend the southern frontier from the continued presence of Spanish colonials in the American Southeast, Fort Frederica on St. Simons Island served as the British military headquarters in colonial America. During its heyday, from 1736 to 1758, General James Oglethorpe's town and fort played a pivotal role in the struggle for empire between the competing interests of England and Spain. This clash of cultures pitted British redcoats, the Highland Independent Company of Foot, and coastal rangers and sympathetic Southeastern Indians against the Spanish forces. They were concentrated beyond "the debatable land" southward to St. Augustine, Florida. Source: http://www.georgiaencyclopedia.org/nge/Article.jsp?id=h-683