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.