title: The Keys to the Kingdom
context: The Keys to the Kingdom is a fantasy–adventure book series written by Garth Nix, published in seven books between 2003 and 2010. The series chronicles the adventures of Arthur Penhaligon, an asthmatic 12-year-old boy who is chosen to become the Rightful Heir of the House, the center of the universe. The core storyline involves Arthur attempting to defeat the Morrow Days, the criminal Trustees of the House. The series takes place over a span of three weeks (time moves at different speeds at different locations in the series).
question: When was The Keys to the Kingdom series first introduced?
answers: 2003