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.
The GAR was bought by a local media production company in 2011, with renovation work currently underway.
Special thanks to reader Buzz who tipped us to this location.