Yes, we could tell you everything there is to know about the Cathedral of Siena. We could say that it was built between 1226 and 1370, that the facade has Gothic influences and that the black and white are the civic coat of arm colours. We can tell you this, and so much more, but sometimes, images are worth a thousand words.
The floor is an artwork on its own. Considered by Giorgio Vasari "the most beautiful ... largest and most magnificent ... that ever was made", it was the result of a plan that took real form over the ages only to see its completion in the 18th century. Here a detail of the emblem of one of the confederate cities surrounding the She Wolf of Siena.
On the left hand nave you can enter the Library, where beautiful and colourful frescoes tell the story of cardinal Enea Silvio Piccolomini, who became Pope Pius II. His nephew Francesco Todeschini Piccolomini, archbishop of Siena and the future pope Pius III, commissioned the library in 1492 as a repository of the books and the manuscript collection of his uncle. Executed between 1502 and 1503 by Pinturicchio and his assistants, the stunning paitnings create a visual journey that is a feast to the eyes.
Well worth the ticket is the Gate of Heaven. You will be taken upstairs, where only the workers looking after the Cathedral were allowed to go. Dedicate few minutes to absorb the golden starry night at the top of the dome's roof.
Make sure to visit either early morning or late afternoon. The light is better, and the crowd not so bad. It's a place speaking centuries of humanity, it deserves time and space to allow all the small details to get in. Only downside, you would want to go back again and again and again... Just in case you missed that little star, at the top of the roof, on the left.