The series is based on a six-volume doujinshi series by Yoshitoshi Abe which was never officially published.
The final volume and complete collection of the series carry a 12 rating in the UK for 'moderate sex references'. This is because of a trailer for Ikki tousen (2003) that is contained on the final disc. The show itself has a PG rating.
Yoshitoshi Abe has cited Haruki Murakami's "Hard Boiled Wonderland and the End of the World" as an influence on the series.