Warning: Spoilers Ahead
Alexis is safe!! Hizzah!! ::throws confetti:: ::blows noisemakers:: ::pops poppers:: She is now back home where she should be. But the fact that Castle now has a much better idea of who his father is (even if we didn't find out if the man eats whip cream out of a spray can, which I kind of needed to be something that they shared), and the kind of danger that he is in just by being related to the man, I can see Castle getting a little bit more overprotective about Alexis. Who wants to bet that he hires some undercover security people to watch over Alexis and make sure that this sort of thing never happens to her again? Of course, she will end up figuring it out after a while, and I'm sure that she will be kind of furious (even while she's also kind of relieved by the idea of having people around who are only there to make sure that she isn't kidnapped again by a psychopath and taken to France).
But it really is gratifying to know that Gramps is watching over Alexis and Castle and Martha the way that he can. So, even if he isn't able to be there with them like he would like to be, he's still a part of their lives in a way ... especially when you consider the significance of that book that he gave to Castle as a kid ... the same book that he sent to Castle to let him know that everything was alright.
It's also really nice to know that the family unit now has some answers that they didn't have before, and that there may be some peace inside of themselves now that they know the things that they didn't know before. After all, it did seem to bother Alexis a bit when she was doing that project about her family tree, and she had nothing to go on for her paternal grandfather and his family. What they have now isn't a whole hell of a lot, but at least now it's something (and something is better than nothing).
And honestly, I can't understand why Castle promised Beckett that he would never do anything like that ever again without her. He made sure to tell her not to make promises that she couldn't keep, and this is something that he is not going to be able to keep at all. As soon as something bad happens to Alexis ... he is going to do whatever he has to do to make sure that she is ok ... even if it means pissing off Beckett (cuz that will only be secondary when it comes to getting back his baby).