The Grand Army of the Republic Building, or GAR as it's known, is a triangular castle located on the corner of Grand River and Cass streets. Built in 1900 for veterans of the Civil War, the building was a fixture of downtown life until it was closed and abadnoned in 1982.
HistoricDetroit.org has an excellent writeup on the history and future of this location.
Special thanks to reader Buzz who tipped us to this location.