Rome's most theatrical Baroque square, built on the footprint of the 1st-century Stadium of Domitian — its elongated shape still traces the ancient racecourse beneath. Three fountains punctuate the piazza; Bernini's Fontana dei Quattro Fiumi (1651), with river gods symbolising the Nile, Ganges, Danube, and Río de la Plata, is the centrepiece.
The square is lined with cafes and street artists and fills with visitors throughout the day; it's most atmospheric in the evening. Don't eat at the restaurants on the piazza itself — overpriced and unremarkable. Sant'Eustachio il Caffè is a five-minute walk away.