The correct answer to this open question is the following.
The number is 4. The former location of Fort Screven was on Tybee Island. It was built in 1897, as part of the defense system located on the coasts of the United States. Originally, the founder of the colony of Georgia, James Oglethorpe gave the order to build this fort to secure the Savannah River. The fort was used by the military until 1947. Today is a great museum visited by many Americans.