The four wall-surfaces that flank the three western doorways are decorated with very beautiful sculpture in relief, once ornamented with colour, the designs for which, according to Burckhardt, must be ascribed to the architect of the whole, though executed by other (but still Sienese, not Pisan) hands.
His family was Sienese in origin, and his father, Colonel Domenico Pecci, had served in the army of Napoleon.
These painters seem to have been influenced by the contemporary masters of the Sienese school.
Among the noblest fruits of Sienese art are the public buildings adorning the city.
Here are Pinturicchio's famous frescoes of scenes from the life of the latter pontiff, and the collection of choir books (supported on sculptured desks) with splendid illuminations by Sienese and other artists.