Castillo de San Marcos United States

Legendary

Explore the oldest masonry fort in the continental United States.

Photo by Matthew Dillon

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Cannonballs slammed into this fort's walls, and instead of cracking them apart, the walls simply swallowed the hit. That's because Castillo de San Marcos was built out of coquina, a strange kind of stone made from millions of tiny compressed seashells.

Spain built the fort to protect St. Augustine, Florida, after pirates raided the town in 1668. Wooden forts had stood there before, but fire, storms, and rot kept destroying them.

Construction began in 1672. Workers, some free and some enslaved, mined the coquina stone from quarries on nearby Anastasia Island and floated it across the bay on rafts.

It took 23 years to finish the fort, completed in 1695. The finished walls stood 33 feet high and up to 14 feet thick at the base.

Coquina turned out to be the perfect material for a fort. Because it's soft and full of tiny air pockets, cannonballs sank into the stone instead of shattering it, protecting the walls from serious damage.

That strength was put to the test in 1702 and again in 1740, when English forces attacked St. Augustine. Both times, the coquina walls held strong, and the fort was never captured in battle.

Over the centuries, the fort changed hands several times as Spain, Britain, and later the United States took turns controlling Florida. It served as a military base, a storage building, and even a prison at different points in its history.

Walk along the fort's thick coquina walls and look out over Matanzas Bay. Watch a cannon-firing demonstration, often held by rangers in period costume. Explore the old gun rooms and dungeons, where soldiers and prisoners once lived and worked.