New Castle is a city in Henry County, Indiana, 44 miles (71 km) east-northeast of Indianapolis, on the Big Blue River. In 1900, 3,406 people lived in the town; in 1910, 9,446; in 1920, 14,458; and in 1940, 16,620. The population was 18,114 at the 2010 census. The city is the county seat of Henry County. New Castle is home to New Castle Fieldhouse, the largest high school gymnasium in the world. The city is surrounded by agricultural land.