Question: This census-designated place first settled in 1623 is home to a defensive fortification built at the mouth of what river?

Context paragraphs:
Fort McClary: Fort McClary is a former defensive fortification of the United States military located along the southern coast of Maine at Kittery Point.  Built at the mouth of the Piscataqua River, it was used primarily throughout the 19th century to protect approaches to the harbor of Portsmouth, New Hampshire, and the U.S. naval shipyard.

Inarajan Pillbox: The Inarajan Pillbox is a World War II-era defensive fortification near the mouth of Inarajan Bay in the United States territory of Guam.  Set on a limestone outcrop on the south side of the bay, the fortification occupies a natural niche in the formation, with a wall of piled coral rocks and cement covering part of the opening.  This wall is about 1.8 m long, 2.0 m high, and 1.0 m thick.  A cement gunport is set in the wall, about 0.75 m above ground level.  This fortification was built by Japanese soldiers occupying Guam between 1941 and 1944.  The structure was listed on the National Register of Historic Places in 1991, as "Inalahan Pillbox".

Newport Barracks, North Carolina: Newport Barracks, North Carolina were located near the town of Shepherdsville (now Newport, North Carolina), near the site of an early defensive fortification built in the 1830s.  The camp was near the tracks of the Atlantic and Coastal Railroad and paralleled a creek that led to the Newport River (which in turn leads to Beaufort Inlet).

Kittery Point, Maine: Kittery Point is a census-designated place (CDP) in the town of Kittery, York County, Maine, United States.  First settled in 1623, Kittery Point traces its history to the first seafarers who colonized the shore of what became Massachusetts Bay Colony and later the State of Maine.  Located beside the Atlantic Ocean, it is home to Fort McClary State Historic Site, and Fort Foster Park on Gerrish Island.  Cutts Island is home to Seapoint Beach and the Brave Boat Harbor Division of the Rachel Carson National Wildlife Refuge.

Talofofo Pillbox: The Talofofo Pillbox is a historic World War II-era defensive fortification in Talofofo, Guam.  It is located near the coast, about 127 m south of the mouth of the Togcha River and 27 m inland from the high-tide line.  It is roughly 2.75 x , built out of concrete and coral limestone.  Its walls are about 0.5 m thick, with an embrasure providing a view of the Togcha River, and a window looking over the coast to the east.  Its entrance is on the landward (south) side.  This structure was built under the direction of the Imperial Japanese Army during its occupation of Guam in 1941-44.

Farmer's Castle: Farmer's Castle was a defensive fortification built opposite the mouth of the Little Kanawha River on the Ohio River by a group of pioneers from the Ohio Company of Associates.  It was located about 15 miles downriver of Marietta, Ohio, the first European-American settlement in the Northwest Territory.  The pioneers had surveyed the land during the winter of 1788-89, and moved from Marietta to their new farms in April 1789.  They called their town Belle-prairie, or modern day Belpre, Ohio.  Adjacent to the island later known as Blennerhasset Island, the settlers began construction of Farmer’s Castle during January 1791 for protection during the Northwest Indian War.

Tokcha' Pillbox: The Tokcha' Pillbox is a Japanese-built World War II-era defensive fortification on the island of Guam.  It is built on a limestone terrace on Togcha Point, about 1 mi north of the Togcha River and 0.7 mi south of the Ylig River.  It is set in a depression excavated from the limestone about 15 m inland from the high tide land and 1 m above sea level.  It is a roughly rectangular structure built out of steel-reinforced concrete filled with coral and beach aggregate.  The entrance is on the north wall, sheltered by a wall of coral blocks, and the gun port is on the south wall.  This structure was built under the direction of the Japanese military during their occupation of the island 1941-44.

Wall of Severus: The Wall of Severus was possibly a defensive fortification built by the Roman Emperor Septimius Severus (reigned AD 193–211) in Britannia during the emperor's campaign there.  Three texts mention the wall:

Seventh Fort: The Seventh Fort or VII Fort is a defensive fortification built in Žaliakalnis district of Kaunas, Lithuania, during implementation of the first phase of the construction of the Kaunas Fortress.  It is located near the Hospital of Lithuanian University of Health Sciences.  This Fort became the last brick fortification of Kaunas Fortress.  The Seventh Fort is close to the central defenses and it is located in the rear of Kaunas Fortress.  The first owner of fort was the 11th company of Kaunas Fortress Artillery (Imperial Russian Army).  In 1915, German Army occupied the Fort without resistance.  The Seventh Fort is one of the best surviving examples of a two-rampart fort, showing the typical design of a 19th-century Russian brick fort.  Many authentic elements survive, such as iron window frames, ammunition warehouse gate fragments.  During the Fort's restoration, a number of authentic interior details (interior decoration, paving, fittings of ammunition storage doors and gate) were found.

Fort Frye: Fort Frye was a triangular-shaped defensive fortification built by a group of pioneers from the Ohio Company of Associates who moved about twenty miles up the Muskingum River from the settlement of Marietta, Ohio to a location near the mouth of Wolf Creek.  During 1789 the pioneers established settlements now known as Waterford and Beverly on the southwest and northeast banks of the Muskingum, respectively.  The settlements were located about 13 miles downriver from a small group of pioneers at Big Bottom.  During January and February 1791, following the massacre at Big Bottom and the start of the Northwest Indian War, the settlers built Fort Frye at Beverly.

Supporting paragraphs: Kittery Point, Maine Fort McClary
Answer: Piscataqua River