The setting for a book is as important to me as characters. Where and when the book takes place can be crucial. For instance, not too likely to have people freezing to death in Florida in August. Conversely, no one is going to die from heat stroke in the Arctic in the winter.
The fortress, the Castillo de San Marcos, is in St. Augustine and has the honor of never being taken by force.
The second part of the story is set on an island that is now New Smyrna Beach (established in 1768). The turtle shaped mound (Turtle Mound) is located at the south end of the island in what is now a national park, Canaveral National Seashore.
It is thought that Turtle Mound was used as a lookout post. When you see the view, you can understand why. These photos show the view of the ocean, but the river is clearly visible from the other side of the mound.