Castillo de San Marcos is a Spanish colonial fort built between 1672 and 1695 from coquina, a shell-stone quarried from nearby Anastasia Island. It is the oldest masonry fort in the contiguous United States and a National Monument managed by the National Park Service. The fort sits at the edge of Matanzas Bay, with sweeping water views from the surrounding grounds.

Leashed dogs are welcome on the exterior grounds, lawn, and moat area surrounding the fort. Dogs are not permitted inside the fort structure or NPS headquarters buildings. The NPS Bark Ranger program is available here: visitors who review the B.A.R.K. principles and take the pledge earn a virtual Bark Ranger badge. Retractable leashes may not be extended beyond six feet. Dogs may not be left unattended or tied to fixed objects.

The park grounds are open daily from 6 am to midnight. The fort interior is ticketed and open daily except Thanksgiving and Christmas, from 9 am to 5:15 pm, with last admission at 5 pm. Paid parking is available at the City of St. Augustine lot in front of the fort.