Most tourists in Siena go to the Palazzo Pubblico or the Duomo. Almost nobody walks into the State Archive on Banchi di Sotto.
I did — because I was researching the Tavolette di Biccherna: painted wooden covers of the city's municipal account books, commissioned continuously from 1258 to the early 17th century. Over 350 years, the city hired painters like Pietro Lorenzetti, Francesco di Giorgio Martini, Sano di Pietro and Vecchietta to decorate their financial records.
What's remarkable is what they painted: not just religious scenes, but political propaganda, eyewitness accounts of earthquakes and naval battles, satirical takes on sumptuary laws, and what amounts to the earliest secular portraiture in Sienese art.
The museum inside the archive is free. Open Saturdays 10am–1pm. Almost no one goes.
I made a documentary about the collection if anyone's interested — happy to share the link https://youtu.be/tRiZIfAC_gQ?is=edyCfu-MHnzq-7PJ