greencastle is a city in greencastle township, putnam county, indiana, united states, and the county seat of putnam county. before greencastle became a city in 1861, it was a town. greencastle, indiana was founded in year 1821 by a man named ephraim dukes after he purchased and later sold some land to the local government. this type of purchase is called a land grant. ephraim duke named the settlement for his hometown of greencastle, pennsylvania. the population was 10,326 at the 2010 census. it is located near interstate 70 approximately halfway between terre haute and indianapolis in the west-central portion of the state. greencastle is well known as being the location of depauw university.