Should you ever wish to build an authentic fort, take notes at Castillo de San Marcos, America's oldest and best-preserved masonry fortification. The structure represents the quintessence of the "bastion system" of fortification. It took 23 years to build, and sprawls over a 20 1/2-acre complex that includes a double drawbridge entrance (the only way in or out) that spans a 40-ft. dry moat. There's also a reconstructed section of the walled defense line that surrounded St. Augustine, incorporating the city's original gate. The Castillo was never captured in battle, and its coquina walls (limestone made from broken seashells and corals) did not crumble when pounded by enemy artillery or violent storms for over 300 years. Today, the old bombproof storerooms surrounding the central plaza have exhibits about the history of the fort, a national monument since 1924. You can tour the vaulted powder magazine, a dank prison cell (supposedly haunted), the chapel, and the guard rooms. Climb the stairs to get a photo op-worthy view of Matanzas Bay. A self-guided tour map and brochure are provided at the ticket booth. If available, the 20- to 30-minute ranger talks are well worth attending. Popular torchlight tours of the fort are offered in winter, and special evening events and historical reenactments take place often. Check the website for more information and schedules. Visits here take an average of 1–2 hrs.