A grand Renaissance palace built in the 1460s for the family of Pope Pius II (Enea Silvio Piccolomini, the most famous Sienese), modelled on Roman models by Bernardo Rossellino. It now houses the State Archives of Siena, with one of the most remarkable surviving city archives in Italy.
The Tavolette di Biccherna — wooden book covers commissioned each year from major Sienese painters to bind the city's account books, from the 13th to the 17th century — are displayed here in a small free museum. Few visitors realise they exist; they are quietly one of the best things to see in Siena.