Here's the dragon I drew to inhabit the castle piece:

It turned out pretty good IMO, even if it could use a little more contrast. But I did try putting in some tighter blacks, and it just ended up looking cartoony. I prefer the more painterly look of this version.
I think what I'd like to do is draw the castle in Colors using the Sketchup one as a reference, then combine the dragon and castle in Photoshop.