Warning: Spoiler Alert
Good on Martha for knowing that Castle and Beckett are seeing each other, even though the two of them have thought that they have been completely secretive and awesome about not letting anybody know what is really going on between them. Also, double good on her for telling him that he needed to tell Alexis about what was going on between the two of them. Honesty with the kids when you are seeing someone new is necessary, or resentments grow out of that (and that's a fact).
Castle really has been waiting a while to be able to say that the butler did it ... ever since season three (with that episode "Lucky Stiff") ... well, at least since then. So, good for him for finally being able to say it and get something off of his bucket list (oh, you know that was on his bucket list). Sad, though, about how this one douche was responsible for the death of an entire family (whether the deaths were accidental or purposeful). But it looks like he's going to have to put "getting the new captain to like me" is going back onto the bucket list. I almost got happy for him when it looked like she was going to like him, but I had the feeling that it wasn't going to last for very long. She's hated him for much too long for the writers to suddenly have her pull a 180 and start loving him for giving her that doll (no matter how hard it was for her to find, and how much she was excited about getting it).
Speaking of the doll, it would have been nice if the writers hadn't made it so obvious that there was something about the doll. Even if you didn't figure out who it was that had actually killed the twins, the fact that such a big deal was made about the doll from the moment it appeared on screen made it clear that there was something significant about it. As soon as it showed up in Esposito's hands, I knew that there was something about the doll that was going to be significant to the rest of the episode, and I kind of wished that at least that part was a little less "in your face". But the part where Castle dropped the dolls on the ground to get to what was inside of the one ... how did the one twin get anything inside of the doll, if Castle had to smash it to bits to get to what was inside of it? If something was put inside of it for safe-keeping, wouldn't it be able to get opened again without breaking it (since I'm assuming that the twins would have needed the glass eye and the flashdrive to prove that the butler was the one that killed their parents)? So, couldn't there have been an easier way than that? And if the captain was playing with the doll when she first got it, wouldn't she have been able to tell that there was something inside of it (after all, she was moving it around, and the stuff inside of it would have had to have moved around as well and made some noise, or something)? I doubt that the eye and the flashdrive were wedged inside of there so tightly that they wouldn't have shifted at all when the doll was moved around. At some point, someone would have had to have noticed that the thing was making noises that it really had no business making.