Bellarion The Fortunate by Rafael Sabatini

Bellarion the Fortunate is a historical novel set at the beginning of the 15th century in northern Italy. 

Bellarion, abandoned as a child and raised in an abbey, departs as a young man with a letter of introduction from the respected abbot, intending to study in Pavia. He meets and travels with a Franciscan friar, but discovers that someone has robbed him of his money and letter. Upon arriving in Casale, the capital of Montferrat, he finds himself pursued by the authorities, who suspect him of complicity with the false friar, actually a well-known scoundrel named Lorenzaccio da Trino.

He flees until he reaches a palace and enters by a garden door which he is surprised to find unlocked. A beautiful lady admits him, bolts the door, and takes pains to hide him from his pursuers. They arrive, and Bellarion listens to her rebuff them, discovering that she is the Princess Valeria of Montferrat...