From NCpedia. By Paul Branch.
The fort was named after North Carolina's first governor and Revolutionary War hero Richard Caswell (1729-1789), and was a permanent masonry garrison fort built by the U.S. Army Corps of Engineers on the eastern end of Oak Island in Brunswick County between 1826 and 1838.
It guarded the mouth of the Cape Fear River as part of a national chain of forts for coastal defense of the United States known as the Third System. The fort was designed by Brigadier General Simon Bernard as an irregular pentagon with a completely encircling outer wall, or covered way, and an interior main work that was loopholed for defense.
The fort was never fully armed, and, until 1861, it was usually occupied by army caretakers, usually a single ordnance sergeant.
Following South Carolina's secession from the Union, fears that the Federal government would send troops to occupy North Carolina's forts, prompted secessionists in the state to seize Fort Caswell on January 9, 1861. But, North Carolina had not yet seceded and the fort was returned to the United States.
However, after war had begun, North Carolina troops again took over Fort Caswell on April 16, 1861.
--Old B-Runner