Complex sentence: Theater am Kärntnertor or Kärntnertortheater was the most prestigious theater in Vienna during the eighteenth and nineteenth centuries, for it served as the "Kaiserliches und Königliches Hoftheater zu Wien", the "Imperial and Royal Court Theater of Vienna".
Split into simpler sentences: Theater am Kärntnertor or Kärntnertortheater was a prestigious theater in Vienna during the eighteenth and nineteenth centuries. Its official title was "Kaiserliches und Königliches Hoftheater zu Wien", the "Imperial and Royal Court Theater of Vienna".