Siena started life as an Etruscan settlement, (c 900-400 BCE) inhabited by a tribe called the Saina. Later, during the reign of the Emperor Augustus (63 BCE to 14 CE), like many towns in Tuscany Siena became a Roman Colony "Saena Julia".
The Siena that we see today started to come into being in the 8th century, when the city was conquered by Charlemagne. The new Frankish lords married into the existing Sienese nobility and founded Abbeys that still stand today, like the beautiful Abbey of Sant'Antimo.
The Frankish connection also developed the pilgrimage and trading routes with Northern Europe. The development of the wool trade and of Siena as a centre for money lending would lead to Siena's golden age, largely considered to be between 1260 and 1348.
As Siena increased in size its peculiar siting and layout became more important: it has no large river running through it or by it, but was founded, presumably for defensive purposes, on a tripartite ridge of tufa rock. The city's ingenious solution to the lack of water was to carve over 25 kms of tunnels underground to collect rain and ground water. You can read about it (and organise a visit to this hidden marvel) here: Bottini in Siena.
As the city's influence increased it came into conflict with neighbouring Florence - also because Siena was harbouring Ghibelline refugees was the city. The two cities settled their differences at Montaperti with Siena's decisive victory in 1260. The name of the battle lives on and will still be hurled at Florentines in arguments, 760 years later!
The victory at Monaperti marked a surge in Siena's fortunes. Many of the cities famous figures and artists date from this period. Duccio di Buoninsegna, Simone Martini, the fabulous Lorenzetti brothers, Ambrogio and Pietro and many more.
In 1348 the Black Death visited Siena and laid waste the population. The city was building a new, larger Cathedral and the work was never finished. You can see what would have been the new nave by the Museo dell'Opera del Duomo and even climb to the top of the tall sliver of brickwork to get a fantastic view of the city.
After this blow to their population and finances, Siena went through a period of decline, relieved only after a century of a succession of councils, alliances and expulsions. The papacy of Enea Silvio Piccolomini marked a rise in prominence of the city - he became Pope Pius II in 1458. One of his legacies was the beautiful town of Pienza (named after himself) where, together with architect Bernardo Rossellino, he started to reshape the town as an ideal city.
By the early 16th century the city was in trouble again. During a period of civil unrest Charles V (Holy Roman Emperor, Archduke of Austria and King of Spain) sent a spanish garrison to take over the city. The Sienese expelled the garrison and Charles laid siege to the city. After 18 months Siena surrendered to Spain - this was the end of the Republic of Siena.