This building is especially dedicated to housing the works of the Cathedral of Siena, in order to promote better conservation. And, indeed, you will find this museum located in the unfinished wing of the Duomo itself. 

Of all the pieces to be found here, the most outstanding are the sculptures by Giovanni Pisano for the façade of the cathedral. And, although they are a bit dilapidated, it is well worth taking a look. 

In addition, in the Galleria delle Statue, you will also find works by Donatello and the "Madonna with Child" by Jacopo della Quercia, one of the most important sculptors of medieval Siena. 

But the most significant creation is to be found on the upper floor of the museum. This is the famous altarpiece "La Maestà" by Duccio. These two panels are so important that the whole room, in which they are located, is devoted to them. They are indeed considered to be a masterpiece of the Sienese school. You can see an image of the Virgin and Child on one of the altarpieces and some scenes from the life of Christ on the other. 

And do not forget to climb to the loggia on the top floor of the museum, where you can enjoy fantastic views of the city.