In the early morning of 24 February 1525, Imperial engineers opened breaches in the walls of Mirabello, allowing Lannoy's forces to enter the park. At the same time, Leyva sortied from Pavia with what remained of the garrison. In the ensuing four-hour battle, the French heavy cavalry, which had proven so effective against the Swiss at Marignano ten years prior, masked its own artillery by a rapid advance and was surrounded and cut apart by landsknechts and d'Avalos's massed Spanish arquebusiers. Meanwhile, a series of protracted infantry engagements resulted in the rout of the Swiss and French infantry. The French suffered massive casualties, losing the majority of their army. Bonnivet, Jacques de la Palice, La Trémoille, and Richard de la Pole were killed, while Anne de Montmorency, Robert de la Marck, and Francis himself were taken prisoner along with a host of lesser nobles. The night following the battle, Francis gave Lannoy a letter to be delivered to his mother in Paris, in which he related what had befallen him: "To inform you of how the rest of my ill-fortune is proceeding, all is lost to me save honour and life, which is safe." Soon afterwards, he finally learned that the Duke of Albany had lost the larger part of his army to attrition and desertion, and had returned to France without ever having reached Naples. The broken remnants of the French forces, aside from a small garrison left to hold the Castel Sforzesco in Milan, retreated across the Alps under the nominal command of Charles IV of Alençon, reaching Lyon by March.