The Knights of the Cross by Henryk Sienkiewicz
The Knights of the Cross tells the story of the young poor nobleman Zbyszko of Bogdaniec, who with his uncle Maćko of Bogdaniec is returning from a war against the order of the Teutonic Knights ("Knights of the Cross") in Lithuania. In a tavern inn he falls in love with the lovely Danusia, who is traveling with the court of the Duchess Anna. He swears to her his knight's oath and promises to bring her "three trophies" from the Teutonic Knights.
However, on his way to the royal city of Cracow, Zbyszko attacks Kuno von Liechtenstein, who is an official diplomatic delegate of the Teutonic Knights. The penalty is death. Yet on the gallows Danusia saves him when she jumps onto the platform in full view of the crowd and promises to marry him, covering his head with a handkerchief (an old Polish tradition that carries with it a stay of execution if the couple wed). Then Zbyszko and Maćko return home to their estate, where they rebuild their mansion. After some time Zbyszek returns to Danuśka and marries her.
But she is soon treacherously kidnapped by four Teutonic Knights, for they want to their revenge...