Depends on the time period. A knights castle would probably focus more on defensive areas and fortifications than a palace or castle from the early modern era, which would focus on representative rooms for the members of the family living there.
But I'm sure you can go by: throne room, dining room, several bed rooms, kitchen, stables, armory, several walking rooms, offices, and for sure some garden (be it a large representative formal garden or just a place to grow herbs and food for the kitchen)
