The first room I would to tell about is the donjon. The prisoners were kept there in the darkest, dampest, deepest part of the castle(or the dungeon). In other rooms, they stored the food for the city. There was almost always a well for water. There were kitchens and pantries to store food for daily use and the great hall and bed chambers were located there. In other words, it is the most used room in the castle. I think it is the most important room in the castle