The outstanding example of a European-style fortress on the North American continent is the Castillo de San Marcos in St. Augustine, begun by the Spanish in the 1670s to protect its treasure fleets plying the nearby Florida Straits. Other impressive examples of colonial forts include Fort King George and Fort Morris in Georgia, the ruins of Fort Frederica on St. Simons Island, and Fort Matanzas, fourteen miles from St. Augustine.
The forts of the young United States evolved slowly in response to threats of war and continuing advances in weapons technology. A national program of fortification building known as the First American System began in the 1790s. Mostly of earthen construction, the First System forts quickly fell into decay after the war scare passed. With renewed threats from Great Britain in 1807, the Second System of fortifications began. Many of these forts were rushed into service during the War of 1812. In the wake of that conflict, the U.S. began its Third System of forts, meant to provide a comprehensive program of coastal defense with advanced armaments.