Complex sentence: But the principal, Frederick Francis II, Grand Duke of Mecklenburg, patron of Warnemünde Church insisted that a single large window would be much more appropriate; the architects designed the rose windows, which can still be seen today.
Split into simpler sentences: But the principal, Frederick Francis II, Grand Duke of Mecklenburg, patron of Warnemünde Church insisted that a single large window would be much more appropriate. Because of "His Highness" suggestion, the architects designed the rose windows, which can still be seen today.