This did feel like one of their solid, convoluted and clever mysteries from when the series was at its peak. The main thing that bugs me (aside from the clock/evidence thing) is Beckett giving up her tradition to be with Castle. What Beckett said about her tradition being to keep watch on Christmas was genuinely moving to me. She chooses to do something meaningful, to try to protect others at Christmas so they don't have to suffer the same loss she did. It makes her like some sort of Christmas Batman. That's completely awesome, and very poignant. I wanted Castle to go to the precinct and spend the evening with her there. And he was going to. And then they went and ruined it by going for the sappier, shallower ending, by having her give up her noble and selfless calling in favor of a big tree and toy trains.