[3] From 1287 to 1355, during the rule of the Noveschi, the Republic experienced a period of great political and economic splendor: new buildings were commissioned, including that of the Cathedral of Siena, the Palazzo Pubblico, and a substantial part of the city walls completed.

[4][5] A combination of economic decline, sparked by the Black Death, and political instability led to its absorption by the rival Duchy of Florence during the Italian War of 1551–1559.

[6] The foundation of the Republic of Siena is traditionally dated to 1125, the year in which the last reigning representative of the Government of Bishops was deposed, at the time at the head of the city and the surrounding countryside.

The bishop was forced to concede a greater say in the running of the city to the nobility in exchange for their help during a territorial dispute with Arezzo, and this started a process which culminated in 1167 when the commune of Siena declared its independence from episcopal control.

In the 13th century, Siena was predominantly Ghibelline, in opposition to Florence's Guelph position (this conflict formed the backdrop for some of Dante Alighieri's Divine Comedy, completed in 1320).

However, the port, swept away during the 14th century by the violent flood that removed the course of the Ombrone from the city, never had any development, also due to the incorrect economic policy of Siena and the lack of a productive background.

In 1355, with the arrival of Charles IV of Luxembourg in the city, the population rose and suppressed the government of the Nove council and expelled any family associated with the Noveschi party.

This form of government was short-lived, soon replaced by the Quindici ("The Fifteen") reformers in 1385, the Dieci ("The Ten", 1386–1387), Undici ("The Eleven", 1388–1398) and the Twelve Priors (1398–1399) who, in the end, gave the city's lordship to Gian Galeazzo Visconti, the Duke of Milan, in order to defend it from Florentine expansionism.

In the wake of more responsibilities from the Church, Raffaello was forced to cede the city to his nephew, Francesco Petrucci, who only ruled for a year before being ousted by Pandolfo's youngest son, Fabio.

With the end of the Petrucci era internal strife resumed, with the popular faction ousting the Noveschi party supported by Pope Clement VII: the latter sent an army, but was defeated at Camollia in 1526.

A notable exception is the hill town of Montepulciano 70 km (43 mi) southeast of Siena, which was claimed by the neighbouring Florence, and was a constant source of hostility between the two city-states.

The mural titled "Allegory of Good Government" (Italian: Allegoria del Buon Governo) painted by Ambrogio Lorenzetti in 1338–39 at the Palazzo Pubblico, Siena's town hall, is considered a magnificent example of civil Gothic art in the world.

New streets were constructed leading to it, and it served as the site of the market and the location of various sporting events (perhaps better thought of as riots, in the fashion of the Florentine football matches that are still practised to this day).

[17] The studium was further supported when, in 1252, Pope Innocent IV declared both its teachers and students exempt from taxes and forced labour levied on their person or property by the city of Siena.

The growth of trade linked to Grosseto salt pans, capable of satisfying the needs of nearby Perugia and neighboring nations, led to the establishment in 1323 of a new organization, called "Monte e Sale", which simultaneously performed the administration of the monopoly and banking duties.

From the end of the 14th century part of the proceeds deriving from the salt monopoly were destined to the payment of rents on the capital lent by state creditors, in order to face the growing public debt of the Republic.

Subsequently, a union of the "Dogane dei paschi" and the salt was also attempted to improve the state financial capacities; however, as early as 1519 the various administrations returned divided.

[20] The vast plots of grazing land in the Valdichiana and the Maremma Senese attracted shepherds from both territories under the direct control of Siena and from neighboring states.

For an efficient exploitation of grazing activity, the Sienese government decided to create a special office, called "Dogana dei paschi", which was responsible for managing the tariff system connected to livestock breeding.