Francesco, Count di Poppi, was in the army of Niccolo, having deserted the Florentines, with whom he was in league, when the enemy entered the Mugello; and though with the intention of securing him as soon as they had an idea of his design, they increased his appointments, and made him commissary over all the places in his vicinity; still, so powerful is the attachment to party, that no benefit or fear could eradicate the affection he bore toward Rinaldo and the late government; so that as soon as he knew Niccolo was at hand he joined him, and with the utmost solicitude entreated him to leave the city and pass into the Casentino, pointing out to him the strength of the country, and how easily he might thence harass his enemies.  Niccolo followed his advice, and arriving in the Casentino, took Romena and Bibbiena, and then pitched his camp before Castel San Niccolo.  This fortress is situated at the foot of the mountains which divide the Casentino from the Val d’Arno; and being in an elevated situation, and well garrisoned, it was difficult to take, though Niccolo, with catapults and other engines, assailed it without intermission.  The siege had continued more than twenty days, during which the Florentines had collected all their forces, having assembled under several leaders, three thousand horse, at Fegghine, commanded by Piero Giampagolo Orsini, their captain, and Neri Capponi and Bernardo de’ Medici, commissaries.  Four messengers, from Castel San Niccolo, were sent to them to entreat succor.  The commissaries having examined the site, found it could not be relieved, except from the Alpine regions, in the direction of the Val d’Arno, the summit of which was more easily attainable by the enemy than by themselves, on account of their greater proximity, and because the Florentines could not approach without observation; so that it would be making a desperate attempt, and might occasion the destruction of the forces.  The commissaries, therefore, commended their fidelity, and ordered that when they could hold out no longer, they should surrender.  Niccolo took the fortress after a siege of thirty-two days; and the loss of so much time, for the attainment of so small an advantage, was the principle cause of the failure of his expedition; for had he remained with his forces near Florence, he would have almost deprived the government of all power to compel the citizens to furnish money:  nor would they so easily have assembled forces and taken other precautions, if the enemy had been close upon them, as they did while he was at a distance.  Besides this, many would have been disposed to quiet their apprehensions of Niccolo, by concluding a peace; particularly, as the contest was likely to be of some duration.  The desire of the Count di Poppi to avenge himself on the inhabitants of San Niccolo, long his enemies, occasioned his advice to Piccinino, who adopted it for the purpose of pleasing him; and this caused the ruin of both.  It seldom happens, that the gratification of private feelings, fails to be injurious to the general convenience.