Town in Italy, in the Cid's time ruled by the prodigious military leader Matilda of Tuscany (1046 - 1115). The reference here is to the loss of Siena by the Spanish to French-sponsored local insurrectionists in August 1552.
When Diego Hurtado de Mendoza was governor for Philip II of Spain, disgruntled citizens of the long-standing Sienese Republic living in Rome entered into negotiations with the agents of the French king. With his assistance they collected men and money, marched on Siena and forced their way in through the Porta Romana on 26 July 1552. The population rose in revolt and drove the Spanish troops into the citadel on 28 July. Cosimo de Medici negotiated a safe conduct and the Spaniards left Siena, rather ingloriously, on the 5 August 1552. The Sienese triumph was short-lived, but Hurtado missed the last act of the drama, having been recalled to Spain in 1554. The forces of Philip II retook the city with the connivance of Cosimo on 17 April 1555. Cosimo purchased the former Sienese Republic from the Spanish emperor, and it remained under Medici rule until 1737.
But before the matter could be proceeded in, the loss of Siena took place, whereupon he was fain to leave Rome; and thus this pious design could not be brought about.