The oldest masonry fort in the United States, the Castillo de San Marcos has exchanged many hands and undergone many name changes throughout the years, but has evolved into a symbol of the cultural clashes that ultimately unify the United States today. Occupying 2.5 acres (101 km²) in downtown St. Augustine, it was first constructed by the Spanish starting in 1672 in order to protect Spanish territories in the New World. Over the next 23 years, the fort was fashioned from a stone called coquina (Spanish for "little shells"), made of ancient shells that have bonded together.
Throughout the years, the fort changed hands between Spanish, British, and American owners, changing its name each time. It also played a vital role in many famous conflicts, from the Civil War to the Spanish-American War. Today, it is a site worth visiting because of its storied history and impressive stature.