well, not knowing the "best" solution, at least when a Castle don't have to be buildt for taking a Catapult shot then one can "cheat" a lot more with construction. I found a homepage once with a company that specialized in building modern castles, have no idea where though, its many years ago. But first thing you'll have to decide as with any building is how big you want to go. You have to think about how many rooms there will be needed, toilet facilities, kitchen facilities, storage, basement (dungeon), etc. Should people be able to live there is also a valid question. The courtyard - how big do you need it, should certain event take place there then you'll have to make room for them. Before you those basic needs set up, you can't start finding plans for it cause you wouldn't know what would be a fit.
Sounds exiting though - so good luck and keep us posted in here ... I'm sure you can find plenty of help here.