Fort William and Mary was a colonial fortification in Britain's worldwide system of defenses, defended by soldiers of the Province of New Hampshire who reported directly to the royal governor. The fort, originally known as "The Castle," was situated on the island of New Castle, New Hampshire, at the mouth of the Piscataqua River estuary. It was renamed Fort William and Mary circa 1692, after the accession of the monarchs William III and Mary II to the British throne. It was captured by Patriot forces, recaptured, and later abandoned by the British in the Revolutionary War. The fort was renamed Fort Constitution in 1808 following rebuilding. The fort was further rebuilt and expanded through 1899 and served actively through World War II.

New Castle is a town in Rockingham County, New Hampshire, United States. The population was 1,000 at the 2020 census. It is the smallest and easternmost town in New Hampshire, and the only one located entirely on islands. It is home to Fort Constitution Historic Site, Fort Stark Historic Site, and the New Castle Common, a 31-acre (13 ha) recreation area on the Atlantic Ocean. New Castle is also home to a United States Coast Guard station, as well as the historic Wentworth by the Sea hotel.