You may see Cathedral of the Holy Savior across town before you actually find this grand building. Look around inside if the doors are open or just admire the facade from the street.
If you love hearing the word "free" as much as most do, you'll be pleased to know this principal church is just that! Cathedral of the Holy Savior is located in San Giovanni. If you want to keep your itinerary full while you're in the area, also drop by Royal Saint Nicholas, Piazza della Repubblica and Museum Satiro.