It's a puzzle to me

In the old days, I'd whack one of these out in a couple of days.  But now I find myself easily distracted, easily frustrated, and needing to go to bed much earlier than I used to.  This one has been at least two weeks in the making.   

Jbird is good at encouraging me into doing puzzles.  He talks about it for a while, gets it out for me, and even finds all the side pieces.  With his busy schedule this semester, he's left me to do most of this one alone.   I've had a picture of this same castle all covered in snow on my desktop for the longest time.  Hopefully we'll visit it some day.

  

I wonder where that missing piece could be?