Castillo de San Marcos is a National Monument. Could it be haunted?
Sources:
Castillo de San Marcos – Old City Ghosts
13 Most Frightfully Haunted Places in Florida – Wander Florida
The Most Haunted Places in Florida – (britonthemove.com)
The Ghosts of Castillo de San Marcos | St. Augustine’s Haunted Fort (ghostcitytours.com)