There is nothing you can not like in the Duomo di Siena. The library Piccolomini is a masterpiece and even those who are not fond of art will drop their mouths once inside.
To me the Duomo di Siena is one of the most beautiful one I have ever seen. The visit will take quite a long time. The floors, with their intricate design, are to die for.
Go for yourself and you will not be disappointed. I Promise.