Home of the French embassy, this formidable Renaissance palazzo, one of Rome's finest, was started in 1514 by Antonio da Sangallo the Younger, continued by Michelangelo and finished by Giacomo della Porta. Inside, it boasts a series of frescoes by Annibale and Agostino Carracci that are said by some to rival Michelangelo's in the Sistine Chapel. The highlight, painted between 1597 and 1608, is the monumental ceiling fresco Amori degli Dei (The Loves of the Gods) in the Galleria dei Carracci.
Visits to the palazzo are by 45-minute guided tour only (in English, French and Italian), for which you'll need to book at least a week in advance – see the website for details. Photo ID is required for entry and children under 10 are not admitted.
The twin fountains in the square outside are enormous granite baths taken from the Terme di Caracalla.