Hidden inside the church is a marvel.  I bet most people don't even know it exists.  You see a modest door that indicates the entrance into "the library".  There is also an admission charge, so you say to yourself: "sure, I'm going to pay a few bucks just to look at a library, right-on, these people saw me coming. . .".  And you walk right by.  

I did walk right by, but a fellow traveler told me to go back.  I discovered a treasure.  Here's the boring story: the place is called Libreria Piccolimini, built in 1495 to house paperwork and books of Pope Pius II.  Here is the exciting part: the frescoes inside (by Pinturicchio) are perfectly (100%) preserved, in vibrant colors and detail.  You can almost smell the paint and mortar.  The walls have not been covered by candle smoke or any other destructive agent, time has stood completely still for over 500 years.  

Bonus #1: in the center of the room the famous Three Graces.  But wait, there's more, bonus #2: the lower part of the walls are covered with beautiful musical manuscripts.

Chances are: you visited Siena, you walked inside the church, but you missed the library.  Tsk, tsk, tsk.  These pictures show you what you missed.