The Castillo de San Marcos National Monument in St. Augustine, Florida, is a historic Spanish fort that's been standing tall for over 450 years. It's the oldest masonry fort in the continental U.S., built to defend Florida and the Atlantic trade route. Visitors can explore its walls, learn about its rich history, and soak up the cultural intersections that have shaped this iconic landmark. You can wander around the outside of the fort for free or pay a small amount to go inside the building. You have to pay for parking here as well but its very close to downtown so its a great base point for your day!