In the following year Michelangelo finished the new sacristy of San Lorenzo, and immediately set to work on the Medicean tombs. But he was constantly worried and interrupted by new commissions from the Pope, who wanted him, among other things, to build a library in which to place the famous collection of books and manuscripts begun by Cosimo de' Medici: "I cannot work at one thing with my hands and at another with my brain!" exclaimed the artist in despair. Nevertheless he undertook to build the library, and carried on both works at the same time, constantly urged on by Pope Clement, who wrote to him in an autograph letter: "Thou knowest that Popes have no long lives, and we cannot yearn more than we do to behold the chapel with the tombs of our kinsmen, and so also the library." These were troublous times for Italy. After the disastrous battle of Pavia, in which he had lost everything "except honour," Francis I. concluded with the Sforza of Milan, with Venice, Florence, and Pope Clement VII. a league against Charles V., which proved fatal to all who took part in it. In 1527, a rabble of German and Spanish soldiers of fortune, led by the renegade Connétable de Bourbon, took and pillaged Rome, and the Pope himself was besieged in the Castle of Saint Angelo for nine months. The Florentines availed themselves of this opportunity to shake off the despotic yoke of the Medici, but two years later, Charles V. concluded the peace of Barcelona with Clement VII., one of the conditions being that he should re-establish the Medicean rule in Florence. But the citizens would not give up their newly-acquired liberty without a struggle, and prepared for a desperate resistance. Michelangelo was appointed Commissary-General of defence, and showed himself worthy of the confidence placed in him by his fellow-citizens. It was in a great measure due to the skill with which he fortified the town, and more especially the hill of San Miniato, that Florence was enabled to withstand the attacks of the Imperial troops for twelve months. But the treachery of Malatesta Baglioni, who commanded the troops of the Republic, paralyzed the efforts of Michelangelo and of its other brave defenders, and in August, 1530, the city fell. Alessandro de' Medici returned in triumph to Florence, and would certainly have beheaded Michelangelo, who only saved himself by hiding in the bell-tower of San Nicolò beyond the Arno, until the first fury of his enemies was over.