During the Medieval period, why was it important for nobles and rulers to build castles?

In the early Medieval period, castles in the true sense of the word, were for defence. Other buildings more like country houses were built where the primary function was accommodation. Not surprisingly these lightly fortified houses were built away from the dangerous frontiers. These days, it's very possible to distinguish between a country mansion and a country cottage. However the differences between a converted farm house and a small mansion might not be so. The same is true with castles. They were all 'individual expressions of ideas', and ruined castles can easily be re-created nowadays to look like the castle once was before it fell apart. (See Source 1). This means that if we so wish, we can fully construct replicas of the original castles so we can determine how noblemen lived in the Middle Ages, etc. The word castle covers a broad area of structures - it is noticeable that in the Domesday Book there are references to strongly fortified houses as well as to castles. Such a dist...