Vintage Color Chrome Postcard. Fort Knox State Park in Prospect, Maine. Eastern Illustrating & Publishing Co. Divided back, unused. Condition: Comments: Fort Knox, now Fort Knox State Park or Fort Knox State Historic Site, in Maine was built from 1844-1869. It is located on the western bank of the Penobscot River in the town of Prospect, Maine, about 5 miles from the mouth of the river. It was the first fort in Maine built of granite (instead of wood). It is named after Henry Knox, the first US Secretary of War, who at the end of his life lived not far away in Thomaston, Maine. The fort was designated a National Historic Landmark in 1970.