Didn't expect the ending either. But it would've been nice to hear Castle clarify things better before he took the ring out. Something like "I will support you wherever you go, whether that's DC or here, I don't care. Because what's important is to have you in my life and make it work matter what, because I love you....." and THEN pop the question.
As it was written it seemed sudden and incomplete...kind of half-hearted.