Charles V now gathered a force of 14,000 German landsknechts and 6,000 Spanish tercios led by Georg Frundsberg and Charles of Bourbon; the forces combined at Piacenza and advanced on Rome. Francesco Guicciardini, now in command of the Papal armies, proved unable to resist them; and when the Duke of Bourbon was killed, the underpaid armies sacked the city, forcing the Pope to take refuge at Castel Sant'Angelo. His escape was made possible by the Swiss Guards' last stand.