Siena, like most hilltop towns of the region, was first settled in the time of the Etruscans. Over the years was embroiled in a fierce rivalry with Florence, which it finally lost largely due to the plague.
I visited Siena in Tuscany on both Globus and Insight tours. We actually stayed for two nights in the old town on our Globus tour which gave me the opportunity to see the place at night.
To see pictures from my 2011 trip to Siena, follow this (link)
or click (here) for a map of Siena.
Globus Tour

Leading up to Siena

The Duomo also known as the "unfinished Cathedral".

Basilica of San Domenico (also called Basilica Cateriniana) holds the remains of St. Catherine

Piazza Salimbeni is one of the oldest banks in Europe

The blue buses of Tuscany
Insight Tour

Saint Catherine

6.50 euro for a Gelato!