The Army engineer in charge during the bulk of the fort's construction was Colonel Sylvanus Thayer, who is best known for his tenure as Superintendent of the United States Military Academy at West Point, New York. It was the fifth largest of the 42 third system forts. The overall plan was pentagonal in shape, slightly irregular to make the best use of the island's terrain. The fort features excellent granite work. A demilune (half-moon) battery protecting the north sally port is a rare feature in US forts. The fort was originally designed for over 200 guns, including some mortars and flank howitzers. During the Civil War it was armed with 15-inch and 10-inch Rodman smoothbore guns.