Location in Garfield County and the state of Colorado. Coordinates: 39°34′20″N 107°32′6″W / 39.57222°N 107.53500°W / 39.57222; -107.53500Coordinates: 39°34′20″N 107°32′6″W / 39.57222°N 107.53500°W / 39.57222; -107.53500. The Town of New Castle is a Home Rule Municipality in Garfield County, Colorado, United States. The population was 1,984 at the 2000 census.