From Birdwatching in New Hampshire , p. 101.

Fort Constitution State Historic Site is located on a peninsula on the northeast corner of New Castle Island. It overlooks both the Piscataqua River and the Atlantic Ocean. The grounds provide a casual gathering spot for picnics. In 1791, the State of New Hampshire gave the United States the neck of land on which Fort William and Mary and a lighthouse were situated. The fort was repaired, renamed Fort Constitution, and garrisoned with a company of United States artillery. Renovations, which included a wall twice as high as that of the colonial fort and new brick buildings, were completed in 1808. It is the ruins of this fort that are seen today.