It's a little over the top, I'll admit. A real "wedding cake" of a cathedral, the Duomo's facade is resplendent with Italian Gothic architectural elements that some might find just a little too rich. But that's perhaps the source of its charm.
The broad piazza surrounding the Duomo of Orvieto provides a wide area from which to stand back and gaze upon the overwhelming beauty and magnificence of this massive structure.
The decorations both inside and out fairly dazzle the senses, whether they be bronze portals by Emilio Greco, ornate carvings, marble sculptures, intricate colorful mosaics, or the famed Last Judgement frescoes by Luca Signorelli.
Even if you're feeling a little "churched out" by your tour of Europe, I recommend taking some time inside the Duomo. It's historically and architecturally important, stunningly beautiful, and one of those not-to-be-missed sights that you will enjoy in retrospect.
Besides, visiting the Duomo gives you an excuse to take the funicular up to Orvieto, a terrifically fun ride that the whole family will enjoy. A bus takes you from the funicular to the central piazza, from which you can easily navigate the town and its wealth of restaurants, pottery shops, and tours of the Underground.
Be sure to check the Duomo's website for hours of operation.