Fort Constitution State Historic Site is located on a peninsula on the northeast corner of New Castle Island. It overlooks both the Pisquatua River and the Atlantic Ocean.
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.