Rafael Sabatini 
Love-at-Arms 

In 15th century Italy, Francesco del Falco agrees to help Valentina, a young woman who decides she’d rather face a literal siege from an unwanted suitor than accept his offer of marriage. Unfortunately, Valentina’s castle is not exactly prepared for action. Francesco makes the best of their limited ammunition and undependable mercenaries, but treachery inside the castle’s walls may prove more of a threat than the soldiers that surround them. (Goodreads)