Castruccio, whose plan of action was fixed, and who well knew what to do, at once attacked the Florentines with five thousand infantry and three thousand horsemen, not allowing them to issue from the river before he charged them; he also sent one thousand light infantry up the river bank, and the same number down the Arno.

The infantry of the Florentines were so much impeded by their arms and the water that they were not able to mount the banks of the river, whilst the cavalry had made the passage of the river more difficult for the others, by reason of the few who had crossed having broken up the bed of the river, and this being deep with mud, many of the horses rolled over with their riders and many of them had stuck so fast that they could not move.

When the Florentine captains saw the difficulties their men were meeting, they withdrew them and moved higher up the river, hoping to find the river bed less treacherous and the banks more adapted for landing.

These men were met at the bank by the forces which Castruccio had already sent forward, who, being light armed with bucklers and javelins in their hands, let fly with tremendous shouts into the faces and bodies of the cavalry.

The horses, alarmed by the noise and the wounds, would not move forward, and trampled each other in great confusion.