The Castillo de San Marcos

And there it is — the Castillo de San Marcos, squatting low and massive on the edge of Matanzas Bay, exactly where it has guarded this town for more than three hundred years. Walk out toward it and take in the shape: four pointed corner bastions in a diamond, a dry moat all around, thick sloping walls. This is the oldest masonry fort in the continental United States, and one of the most remarkable military structures anywhere in the country.

The Spanish began building it in sixteen seventy-two and finished it in sixteen ninety-five, after their wooden forts kept burning or rotting away. And they built it out of the strangest, most wonderful material — a stone called coquina, which they quarried right across the bay on Anastasia Island. Get close to the wall and look: it isn't granite or brick. It's millions of tiny seashells and grains of sand, cemented together by time into a soft, pocked, sandy-colored rock. It looks about as tough as a stale granola bar. And yet this humble shell-stone turned out to be one of the finest fortress materials ever discovered — Boone will tell you exactly why, because the story of what cannonballs did when they hit this wall is one you have to hear standing right here.

Boone can tell you the secret of that shell-stone while you look. Then, when you're ready, head back west to the City Gate — those two weathered stone pillars — and turn south into Saint George Street, the narrow walking lane that runs down the spine of the old town. Follow it south.