Yes, THAT Siena. The one in Italy! I recently went there (virtually) with Smithsonian Associates. Louisa went with me. We were only there a little over an hour, but it was a wonderful escape. The tour guide, Elaine Ruffolo, knows a lot about Italian history and so we learned a lot.
Siena was an Etruscan village and not too important until about 1000 AD. At that time, innovations in farming included how plows were used and oxen were yoked and those improvements led to agricultural surpluses. Those surpluses created opportunities for trade and for banking. The banks were successful enough that they even minted their own coins. It also was a stop on the famous pilgrimage trail Via Francigena going to Rome so there was commerce related to those travelers.
As Siena flourished, so did Florence and a rivalry developed between the two cities. Siena was aligned at that time with the Holy Roman Empire and Florence was then aligned with the Papal State. In 1260, there was a famous battle, Montaparte, between the two cities. Going into that battle, the citizens of Siena appealed to the Virgin Mary for support and left keys to the city on the altar of the church for her. They were at a disadvantage, but they won. According to our guide, citizens of the two cities still bristle at that rivalry today. The people of Sienna credited the Mary for their success and built a beautiful church in her honor, as shown in the photo above. The tower was built a little later and is very distinctive as it is striped in black and white, which are the colors of the Siena flag: