Castillo de San Marcos

After pirates nearly overran St. Augustine in 1668, Queen Mariana of Spain ordered a proper stone fort built to defend the colony; construction ran from 1672 to 1695, using more than 400,000 hand-cut blocks of coquina, a shell-based local stone whose ability to absorb cannon fire rather than shatter proved decisive in later sieges. Convicts and soldiers brought from Cuba worked alongside Spanish military engineers, and some died during construction from disease, accidents, and attack; the fort later served as a prison and witnessed centuries of siege and warfare under five different flags. Now a National Monument, it's considered one of the most haunted sites in St. Augustine. A night watchman has described a young woman in a white dress wandering the grounds before vanishing behind the walls, and visitors report the apparition of a young Spanish soldier searching the outer grounds — local history holds he was killed there by cannon fire while looking for a lost ring.