|Anime: not known for its realism, but it looks great.|
A History of Private Life has been around for a long time and you ought to be able to find it in your public library. It's on the dry, academic side, but if you only read one volume of it, read Volume II. And if you can only stand to read one chapter in Volume II, read "Civilizing the Fortress: Eleventh to Thirteenth Century." (Though it would be a good idea to read the next chapter after concerning the fourteenth and fifteenth centuries too)
|Langley Castle, the finest in 1350's|
The books aren't about castles per se, it's about how (European) people thought of their privacy and by extension their homes, how they organized them... well, as much as we can piece together in hindsight, at least. Castles were one of many stages in that progression.
So the main question about a fantasy castle is: what is its primary function? Fighting off enemy armies? Hosting huge parties? Protecting the town below from the magical laboratory inside it? (That would be interesting, wouldn't it? If you build one, I'd love to hear about it.)
If your castle is primarily for military use, it's a good idea to do some research on lines of fire, controlling access to the gates, and traps like murder holes. If your castle is primarily for parties and social functions, you'll want to think about how to manage lots of people moving around inside it -- both guests and the servants bringing them refreshments.
If it's somewhere in between, as it often is, how does this impact the people living there? I know I read about how castle stairwells were tight and spiraled in a particular direction to give defenders the advantage -- but then I tried to move a mattress up my straight, simple, poorly defensible staircase. How would you do that on a tight, twisty stair?
If the tea's hot when you leave the kitchen but you have to carry it up three staircases and down two hundred feet of hallways, will the king ever have a hot cup of tea?
Here are some other real-world questions that castles need to worry about: where does their drinking water come from? How do they deal with sewage? How many ways in and out are there? (Note that the more doors there are, the less secure the place is.) What would happen if a fire broke out inside?
What are some of your favorite fantasy castles? What purpose do they serve in your story?