Castillo de San Marcos

Castillo de San Marcos is the oldest masonry fort in the continental United States, a Spanish stronghold that has guarded St. Augustine, Florida, since the 17th century. Built of coquina — a soft local shellstone that absorbed cannon fire rather than shattering — the star-shaped fortress was never taken by force in more than 250 years of sieges and changing flags. It stands today as the enduring symbol of Spain's long presence in early America.

St. Augustine, founded by the Spanish in 1565, is the oldest continuously inhabited European settlement in the continental United States, but for a century it was defended only by a succession of wooden forts. After English raids exposed their weakness, Spain began building a permanent stone fortress in 1672. Completed in 1695, the Castillo de San Marcos was constructed from coquina, a soft limestone of compressed seashells quarried nearby on Anastasia Island.

That humble material proved remarkably effective: rather than shattering under bombardment, the porous coquina absorbed and trapped cannonballs. The fort withstood repeated attacks, including major English sieges led by James Moore in 1702 and Georgia's founder James Oglethorpe in 1740, and it was never taken by force. Control of the Castillo instead passed by treaty — to Britain in 1763, back to Spain in 1783, and finally to the United States in 1821.

Renamed Fort Marion by the Americans, the fortress later served as a military prison, holding Native American captives — among them Apache and Plains leaders — during the wars of the 1870s and 1880s. Its original Spanish name was restored in 1942. Designated a national monument in 1924 and administered by the National Park Service, the Castillo remains the oldest and best-preserved Spanish colonial fortification in the continental U.S.