I agree, in my opinion it looks too dark, like it's some sort of a background for something. Adding some form of an accent would be great and get more attention. But maybe that was actually what you were going for, since it's a dark castle after all 
I think it looks fine without the moon. The whole scene appears to be lit from out of canvas to the left anyway so I think you'd get conflicts with lighting and scene composition were you to add it.
IMHO, if you just put one little tiny bright glowing spot somewhere as a cold distant lonely light in the castle... It will make the picture so alive and much more interesting. But it's just my humble proposal. I like that castle very much anyway because of incredible light lying around.
Well, no moon means a much darker castle. :) It also makes sense that the moon would be covered by the snow clouds above.
moody landscape!