Complex sentence: When Napoleon staged a "coup d'état" to assume the status of an Emperor in December 1852, he began to transform Paris into a more open, healthier, and more beautiful city.
Split into simpler sentences: When Napoleon staged a "coup d'état" to assume the status of an Emperor in December 1852, he envisaged a transformed Paris. He decided to remake it into a more open, healthier, and more beautiful city.