Greencastleis a city inGreencastle Township,Putnam County,Indiana,United States, and thecounty seatof Putnam County.Founded in 1821 by Ephraim Dukes on a land grant, Greencastle was a village or town operating under authority of the Putnam County commissioners until March 9, 1849, when it became a town by special act of the local legislature.The city became thecounty seatof Putnam County. Located nearInterstate 70,Greencastle is also well known as being the location ofDePauw University.