5. Castillo de San Marcos, Florida
Overlooking the Matanzas Bay, Castillo de San Marcos is the oldest masonry fort in the continental United States. Built by the Spanish in the late 1600s to protect St. Augustine, its massive coquina walls—crafted from local shellstone—proved nearly impervious to cannon fire. For centuries, the fort changed hands between Spanish, British, and American forces, each leaving their mark on its enduring structure. Today, Castillo de San Marcos stands as a beautifully preserved monument, offering a tangible link to the continent’s earliest colonial rivalries and architectural ingenuity.
