Overnight stay in a Castle?

I am looking for a castle in Vienna, Prague, Dresden or Zurich that we can stay overnight in. We have been to Europe on two prior occasions and my wife has always want to stay overnight in a castle. I have not been able to full her dream, but I have one more chance. I am looking for a real castle, not a building in the city or in the county that has a castle in its name, A REAL CASTLE! Towers and all. We will be traveling May/June. Any help would be appreciated. Thank you,
Mike