Trevi Fountain

Built into the back of the Palazzo Poli and completed in 1762 to Nicola Salvi's design, the Trevi Fountain channels water from the ancient Aqua Virgo aqueduct through a theatrical sculpture of Oceanus riding a shell chariot flanked by tritons and sea-horses. It's the terminus of a Roman aqueduct still in use after roughly two thousand years, and the tradition of tossing a coin over your shoulder to guarantee a return to Rome (immortalized in La Dolce Vita) keeps the piazza packed at all hours.

The scale surprises first-time visitors, since it fills an entire small square rather than sitting as a modest feature. It's a five-minute stop for most, but a magnet regardless — expect a crowd no matter when you show up.