Fort Mantanzas is located 14 miles south of St. Augustine, FL. In order to understand Fort Mantanzas it is helpful to understand The Castillo de San Marcos in St. Augustine, because Mantanzas was built (1740) to protect St. Augustine from sneaky attacks by the British and French from the inlet to the south on the south end of Anastasia Island. So now, you will have to read the next blog post on Castillo de San Marcos, won't you?
By fort standards, it's tiny and kinda funny looking, only 50' long on each side; but situated in the middle of the inlet so no ship could possibly get by it undetected.
Today the National Monument is a valuable ecosystem and home to many threatened and endangered species of animals. It's a lovely place to visit.