Whilst Castruccio made peace with the Florentines, and strengthened his position in Lucca, he neglected no opportunity, short of open war, of increasing his importance elsewhere. It appeared to him that if he could get possession of Pistoia, he would have one foot in Florence, which was his great desire. He, therefore, in various ways made friends with the mountaineers, and worked matters so in Pistoia that both parties confided their secrets to him. Pistoia was divided, as it always had been, into the Bianchi and Neri parties; the head of the Bianchi was Bastiano di Possente, and of the Neri, Jacopo da Gia. Each of these men held secret communications with Castruccio, and each desired to drive the other out of the city; and, after many threatenings, they came to blows. Jacopo fortified himself at the Florentine gate, Bastiano at that of the Lucchese side of the city; both trusted more in Castruccio than in the Florentines, because they believed that Castruccio was far more ready and willing to fight than the Florentines, and they both sent to him for assistance. He gave promises to both, saying to Bastiano that he would come in person, and to Jacopo that he would send his pupil, Pagolo Guinigi. At the appointed time he sent forward Pagolo by way of Pisa, and went himself direct to Pistoia; at midnight both of them met outside the city, and both were admitted as friends. Thus the two leaders entered, and at a signal given by Castruccio, one killed Jacopo da Gia, and the other Bastiano di Possente, and both took prisoners or killed the partisans of either faction. Without further opposition Pistoia passed into the hands of Castruccio, who, having forced the Signoria to leave the palace, compelled the people to yield obedience to him, making them many promises and remitting their old debts. The countryside flocked to the city to see the new prince, and all were filled with hope and quickly settled down, influenced in a great measure by his great valour.
About this time great disturbances arose in Rome, owing to the dearness of living which was caused by the absence of the pontiff at Avignon. The German governor, Enrico, was much blamed for what happened—murders and tumults following each other daily, without his being able to put an end to them. This caused Enrico much anxiety lest the Romans should call in Ruberto, the King of Naples, who would drive the Germans out of the city, and bring back the Pope. Having no nearer friend to whom he could apply for help than Castruccio, he sent to him, begging him not only to give him assistance, but also to come in person to Rome. Castruccio considered that he ought not to hesitate to render the emperor this service, because he believed that he himself would not be safe if at any time the emperor ceased to hold Rome. Leaving Pagolo Guinigi in command at Lucca, Castruccio set out for Rome with six hundred horsemen, where he was received by Enrico with the greatest distinction. In a short time the presence of Castruccio obtained such respect for the emperor that, without bloodshed or violence, good order was restored, chiefly by reason of Castruccio having sent by sea from the country round Pisa large quantities of corn, and thus removed the source of the trouble. When he had chastised some of the Roman leaders, and admonished others, voluntary obedience was rendered to Enrico. Castruccio received many honours, and was made a Roman senator. This dignity was assumed with the greatest pomp, Castruccio being clothed in a brocaded toga, which had the following words embroidered on its front: "I am what God wills." Whilst on the back was: "What God desires shall be."
During this time the Florentines, who were much enraged that Castruccio should have seized Pistoia during the truce, considered how they could tempt the city to rebel, to do which they thought would not be difficult in his absence. Among the exiled Pistoians in Florence were Baldo Cecchi and Jacopo Baldini, both men of leading and ready to face danger. These men kept up communications with their friends in Pistoia, and with the aid of the Florentines entered the city by night, and after driving out some of Castruccio's officials and partisans, and killing others, they restored the city to its freedom. The news of this greatly angered Castruccio, and taking leave of Enrico, he pressed on in great haste to Pistoia. When the Florentines heard of his return, knowing that he would lose no time, they decided to intercept him with their forces in the Val di Nievole, under the belief that by doing so they would cut off his road to Pistoia. Assembling a great army of the supporters of the Guelph cause, the Florentines entered the Pistoian territories. On the other hand, Castruccio reached Montecarlo with his army; and having heard where the Florentines' lay, he decided not to encounter it in the plains of Pistoia, nor to await it in the plains of Pescia, but, as far as he possibly could, to attack it boldly in the Pass of Serravalle. He believed that if he succeeded in this design, victory was assured, although he was informed that the Florentines had thirty thousand men, whilst he had only twelve thousand. Although he had every confidence in his own abilities and the valour of his troops, yet he hesitated to attack his enemy in the open lest he should be overwhelmed by numbers. Serravalle is a castle between Pescia and Pistoia, situated on a hill which blocks the Val di Nievole, not in the exact pass, but about a bowshot beyond; the pass itself is in places narrow and steep, whilst in general it ascends gently, but is still narrow, especially at the summit where the waters divide, so that twenty men side by side could hold it. The lord of Serravalle was Manfred, a German, who, before Castruccio became lord of Pistoia, had been allowed to remain in possession of the castle, it being common to the Lucchese and the Pistoians, and unclaimed by either—neither of them wishing to displace Manfred as long as he kept his promise of neutrality, and came under obligations to no one. For these reasons, and also because the castle was well fortified, he had always been able to maintain his position. It was here that Castruccio had determined to fall upon his enemy, for here his few men would have the advantage, and there was no fear lest, seeing the large masses of the hostile force before they became engaged, they should not stand. As soon as this trouble with Florence arose, Castruccio saw the immense advantage which possession of this castle would give him, and having an intimate friendship with a resident in the castle, he managed matters so with him that four hundred of his men were to be admitted into the castle the night before the attack on the Florentines, and the castellan put to death.
Castruccio, having prepared everything, had now to encourage the Florentines to persist in their desire to carry the seat of war away from Pistoia into the Val di Nievole, therefore he did not move his army from Montecarlo. Thus the Florentines hurried on until they reached their encampment under Serravalle, intending to cross the hill on the following morning. In the meantime, Castruccio had seized the castle at night, had also moved his army from Montecarlo, and marching from thence at midnight in dead silence, had reached the foot of Serravalle: thus he and the Florentines commenced the ascent of the hill at the same time in the morning. Castruccio sent forward his infantry by the main road, and a troop of four hundred horsemen by a path on the left towards the castle. The Florentines sent forward four hundred cavalry ahead of their army which was following, never expecting to find Castruccio in possession of the hill, nor were they aware of his having seized the castle. Thus it happened that the Florentine horsemen mounting the hill were completely taken by surprise when they discovered the infantry of Castruccio, and so close were they upon it they had scarcely time to pull down their visors. It was a case of unready soldiers being attacked by ready, and they were assailed with such vigour that with difficulty they could hold their own, although some few of them got through. When the noise of the fighting reached the Florentine camp below, it was filled with confusion. The cavalry and infantry became inextricably mixed: the captains were unable to get their men either backward or forward, owing to the narrowness of the pass, and amid all this tumult no one knew what ought to be done or what could be done. In a short time the cavalry who were engaged with the enemy's infantry were scattered or killed without having made any effective defence because of their unfortunate position, although in sheer desperation they had offered a stout resistance. Retreat had been impossible, with the mountains on both flanks, whilst in front were their enemies, and in the rear their friends. When Castruccio saw that his men were unable to strike a decisive blow at the enemy and put them to flight, he sent one thousand infantrymen round by the castle, with orders to join the four hundred horsemen he had previously dispatched there, and commanded the whole force to fall upon the flank of the enemy. These orders they carried out with such fury that the Florentines could not sustain the attack, but gave way, and were soon in full retreat—conquered more by their unfortunate position than by the valour of their enemy. Those in the rear turned towards Pistoia, and spread through the plains, each man seeking only his own safety. The defeat was complete and very sanguinary. Many captains were taken prisoners, among whom were Bandini dei Rossi, Francesco Brunelleschi, and Giovanni della Tosa, all Florentine noblemen, with many Tuscans and Neapolitans who fought on the Florentine side, having been sent by King Ruberto to assist the Guelphs. Immediately the Pistoians heard of this defeat they drove out the friends of the Guelphs, and surrendered to Castruccio. He was not content with occupying Prato and all the castles on the plains on both sides of the Arno, but marched his army into the plain of Peretola, about two miles from Florence. Here he remained many days, dividing the spoils, and celebrating his victory with feasts and games, holding horse races, and foot races for men and women. He also struck medals in commemoration of the defeat of the Florentines. He endeavoured to corrupt some of the citizens of Florence, who were to open the city gates at night; but the conspiracy was discovered, and the participators in it taken and beheaded, among whom were Tommaso Lupacci and Lambertuccio Frescobaldi. This defeat caused the Florentines great anxiety, and despairing of preserving their liberty, they sent envoys to King Ruberto of Naples, offering him the dominion of their city; and he, knowing of what immense importance the maintenance of the Guelph cause was to him, accepted it. He agreed with the Florentines to receive from them a yearly tribute of two hundred thousand florins, and he send his son Carlo to Florence with four thousand horsemen.