The oldest city in the United States, St. Augustine, Florida, was founded in 1565 and grew up around the country’s oldest masonry fortification: Castillo de San Marcos. While other wooden fortifications preceded Castillo de San Marcos, the surviving 17th-century fortification is what motivated the true growth of the Atlantic-coast city.
After a 1668 raid by the English pirate Robert Searle destroyed much of St. Augustine, the governor of Spanish Florida, Francisco de la Guerra y de la Vega, ordered a new fort be built—not of wood but of coquina, which is a type of rock made of millions of compressed seashells. This uniquely Floridian material proved highly durable, absorbing the bombardment of cannon balls. Adding to the fort’s sturdiness was the mortar, which was made of oyster shells heated in kilns. The entire fort was coated in a lime stucco, made of sand, water, and lime.
