"I think most castles were built mainly as status symbols, and not the result of a regional or national defense strategy." I think you are mixing up more than

"I think most castles were built mainly as status symbols, and not the result of a regional or national defense strategy." I think you are mixing up more than 1000 years of architectural history in a few lines. Medieval castles were very much defensive installations, which is clear from their a) layout, b) locations, c) the fact that medieval history of warfare is chock-full of castle sieges, while pitched battles like Crécy or Grunwald were spectacular, but rare. Fancy 19th century castles (re-)built in opulent style were mostly conspicuous consumption, as you say. But by that time, nobility preferred chateaus/palaces anyway. Much more comfortable to live in.