I went to Fort Jefferson, located in Monroe County, on 6/17/2017.
According to the ghosttown.com website,
Originally designed as a fort to hold 1500 soldiers, this fort covers almost the entire island of Dry Tortugas. Its walls were 50 feet high and had places for 450 cannons. The Union army occupied it as of 1861 and when the Civil war ended in 1865, the fort was no longer needed. It was turned into a prison not unlike alkatraz and served as such until 1874 when a hurricane damaged it.