Complex sentence: The film has as its basis the murder case involving William "Duff" Armstrong, which took place in 1858 at the courthouse in Beardstown, Illinois--the only courthouse where Lincoln practiced law that is still in use.
Split into simpler sentences: The film has as its basis the murder case involving William "Duff" Armstrong, which took place in Beardstown, Illinois in 1858. It is the only courthouse where Lincoln practiced law that is still in use.