The first two seasons of Castle were very good, laden with provocative humour and plenty of action, but the third season takes it up another gear from the “lost bet” in the first episode to the stunning conclusion in Knockout that had fans of the show gasping in disbelief and salivating for the fourth season to begin (which won't air in New Zealand for at least another couple of months I've heard). The writers of the show have done an exceptional job progressing the relationship between Castle and Beckett and that relationship has come a long way from where they were in the pilot episode. The supporting cast also deserve a mention as they compliment the two leads very well and add to what makes this show so great.
One final thought. R.I.P. to Stephen J. Cannell (one of the real-life writers who made cameo appearances on the show and who was an executive producer) who passed away during the third season last year.