There's been a church on this spot since the late 5th century, but what you see today is much younger thanks to the fire of 1662, which ravaged much of the medieval town, including the cathedral. The rebuilding contract went to a team of Italians, notably the architect Carlo Lurago and the stucco master Giovanni Battista Carlone. The result is a top-heavy baroque interior with a posse of saints and cherubs gazing down at the congregation from countless cornices and capitals.
The building's acoustics are perfect for its main attraction, the world's largest organ, which perches above the main entrance. This monster of a wind instrument contains an astonishing 17,974 pipes and it's an amazing acoustic experience to hear it in full puff. Half-hour organ recitals take place at noon daily Monday to Saturday (adult/child €5/2) and at 7.30pm on Thursday (adult/child €10/5) from May to October and for a week around Christmas. Show up at least 30 minutes early to ensure you bag a seat.