Complex sentence: In the movie's backstory, human civilizations built flying cities, later destroyed by an unspecified catastrophe, forcing the survivors to live on the ground while the sole exception Laputa remains in the sky, concealed by a thunderstorm.
Split into simpler sentences: In the movie's backstory, human civilizations built flying cities, which were destroyed during an unspecified catastrophe, forcing the survivors to live on the ground as before. Just one city, Laputa, remains in the sky, concealed by a thunderstorm.