Siena and the surrounding territory that it knew prosperity. After the Lombard occupation, the old Roman roads of Via Aurelia and the Via Cassia passed through areas exposed to Byzantine raids, so the Lombards rerouted much of their trade between the Lombards' northern possessions and Rome along a more secure road through Siena. Siena prospered as a trading post, and the constant streams of pilgrims passing to and from Rome provided a valuable source of income in the centuries to come. Middle Ages The oldest aristocratic families in Siena date their line to the Lombards' surrender in 774 to Charlemagne. At this point, the city was inundated with a swarm of Frankish overseers who married into the existing Sienese nobility and left a legacy that can be seen in the abbeys they founded throughout the Sienese territory. Feudal power waned, however, and by the death of Countess Matilda in 1115 the border territory of the March of Tuscany which had been under the control of her family, the Canossa, broke up into several autonomous regions. This ultimately resulted in the creation of the Republic of Siena. The Republic existed for over four hundred years, from the 12th century until 1555. During the golden age of Siena before the Black Death in 1348, the city was home to 50,000 people. A major economic centre and among the most important cities in Europe, as well as the main political, economic, and artistic rival of its neighboring city of Florence. In the Italian War of 1551–59, the republic was defeated by the rival Florence in alliance with the Spanish crown. After 18 months of resistance, Siena surrendered to Spain on 17 April 1555, marking the end of the republic. Medicean period After the fall of the Republic, a few Sienese led by the Florentine exile Piero Strozzi, not wanting to accept the fall of the Republic, took refuge in Montalcino, creating the Republic of Siena sheltered in Montalcino. It lived until 31 May 1559 when it was betrayed by the French allies, whom Siena had always supported, concluding with the Peace of Cateau Cambrésis with Charles V, which effectively ceded the Republic to the Medici. The House of Medici, apart from the brief parenthesis of Ferdinando I, who tried to create an organized state, were not able to give a stable structure to the Grand Duchy of Tuscany, keeping almost unchanged the division