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.