New Castle is a town in Rockingham County, New Hampshire, United States. It is the smallest and easternmost town in New Hampshire, and the only one located entirely on islands. New Castle occupies an archipelago, consisting of one main island (Great Island) and several smaller islands surrounded by the Piscataqua River and Atlantic Ocean. The highest point of land is located at The Wentworth-by-the-Sea Hotel, where the elevation reaches 60 feet (18 m) above sea level.
The main island on which the town sits is the largest of several at the mouth of the Piscataqua River and was originally called Great Island. A new route to the island was created in 1821 when three bridges (now two bridges and a causeway) connected Frame Point in Portsmouth with the northwestern corner of Great Island. The community was then an overlooked fishing village, which helped preserve its colonial architecture. However, in 1874 the Hotel Wentworth was built atop a hill with a view of Little Harbor and the ocean. It became the area’s most fashionable resort, growing in size until it was a village unto itself.
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.