The Castillo de San Marco is one of the top attractions of Saint Augustine, the oldest city in the U.S. The fortress, built by the Spanish, is the oldest masonry fort in the country and according to its history, is invincible.

The visit to the interior of the fortress is a must. Walk across the deck of the cannons and imagine the battles that marked the thick walls of coquina, a type of limestone rock formed by corals, shells and compacted sand.

The Castillo de San Marco is a National Monument, and the visit costs 15 dollars per person (the ticket is valid for 7 consecutive days). Admission is free for those who have America The Beautiful, the annual American National Parks pass.