The Piccolomini Library in Siena Cathedral is a hidden gem of Renaissance art and history. Built in 1492, the library boasts exquisite frescos by artist Pinturicchio, depicting scenes from the life of Pope Pius II. The library was meant to house the pope's manuscripts, but they're scattered now. Don't miss the stunning ceiling and rare books on display. It's like stepping into a time machine back to the glory days of the Italian Renaissance!