The anime has a darker tone, whereas the manga, at least initially, tends toward a lighter, more comedic tone. No official English translations exist to date for the manga or its adaptations.The setting resembles a medieval Europe judging by the architecture, the way people are dressed, and the local environment shown in the background. The world in which the story takes place is like an alternate universe in which, chronologically, many things do not makes sense. In fact, despite the medieval feeling with demons and magic, there are also technologically super advanced flying battleships and carriers, tanks and cities made completely of metal platings with siege machines. Also, the pieces played by Hamel and Raiel on their instruments are actually real pieces composed by real historical composers such as Beethoven, Tchaikovsky, Schumann, Schubert, Mozart, Haydn, etc. despite many of these figures existing much later in history.

The world they live in is referred to as Europe, however, the map shown has absolutely no resemblance to Europe at all. Most countries and characters are named after musical themes, be they instruments, tones, beats, etc. There is presence of both fantasy and science fiction elements.The world they currently live in is dominated by two races, the human race and the mazoku race. The mazoku are demonic creatures that once ruled their world, until they were all sealed away into a tiny box by a heroic angel 500 years ago. An age of prosperity and peace arose and human civilization blossomed. Cities were built and nations were created. Science advanced in some, while magic became the source of power for others. One day, a woman by the name of Pandora accidentally opens the box containing all the demons, releasing them all from their imprisonment, darkening the skies and dooming human race to extinction at the hands of the mazoku. In a desperate attempt to save humanity, the Demon King Chestra, who was also released from that box, was sealed again