New Castle is a city in New Castle County, Delaware, six miles (10 km) south of Wilmington, situated on the Delaware River. In 1900, 3,380 people lived here; in 1910, 3,351. According to the 2010 Census, the population of the city is 5,285. According to the United States Census Bureau, the city has a total area of 3.2 square miles (8.2 km²), of which, 3.0 square miles (7.9 km²) of it is land and 0.1 square miles (0.3 km²) of it (3.79%) is water.
New Castle was originally settled by the Dutch West India Company in 1651, under the leadership of Peter Stuyvesant, on the site of a former aboriginal village, "Tomakonck" ("Place of the Beaver"), to assert their claim to the area based on a prior agreement with the aboriginal inhabitants of the area. The Dutch originally named the settlement Fort Casimir, but this was changed to Fort Trinity (Swedish: Trefaldighet) following its seizure by the colony of New Sweden on Trinity Sunday, 1654. The Dutch conquered the entire colony of New Sweden the following year and rechristened the fort Nieuw Amstel ("New Amstel").
New Castle is served by the Colonial School District and the New Castle Public Library. Private schools located in New Castle include Serviam Girls Academy and Tall Oaks Classical School.