At first I have to say... wow! This map is brilliant. I love it. Ah and btw thx for the short tut on making trees.
Now I also have some ideas to fill the spare rooms:
Depedning on whether you this is supposed to be a medieval, modern or fantasy castle you might want to add a chapel for the stuff. The stuff might even be socially divided so there may be some rooms for lower servants (e.g. stableman, maidservants etc.) and some rooms, maybe in the other wing for higher servants (e.g. scribes, scholars etc.)
maybe you want to add some storing rooms next to the kitchen or even have several kitchens (as they often divided the kitchen into several rooms in the middle ages, so they had a bakery, a soup kitchen, a cookshop etc.).
Only suggestions of an unworthy, as always