The Castillo de San Marcos has protected the residents of St. Augustine for more than 300 years. Built using a local shell rock called coquina, this fortress was never captured in battle. Instead of shattering its walls, cannon balls either bounced off or were absorbed into them like in a sponge.
Today it is one of our national treasures maintained by the National Park Service and open to visitors daily. To learn more, visit the Castillo’s site at nps.gov.