Swordsman of Siena - by Baccio Bandini (1962)

A Spanish overlord controls 16th-century Tuscany. He plans to marry into a prominent Italian family and hires an Englishman to serve as bodyguard for his less-than-eager fiancee. The bodyguard finds his allegiance switching to the rebels who seek to overthrow the Spaniard, even as he finds himself drawn to the woman he's been hired to protect.