4. Castillo de San Marcos, St Augustine, Florida
Spanish engineer Ignacio Daza built the Castillo de San Marcos in 1672. Today, it’s known as the most significant and oldest masonry fort in the United States. The Spanish built the fort for the protection of European settlers. The fort wasn’t constructed out of stone but from ancient shells that created a rock over time.
The National Park Service took over the fort in 1933. In 1966, the fort became a National Monument and received a spot on the National Register of Historic Places list. You can see the fort on any day of the year, except on Christmas day.
