This Naldo was one of the best reputed captains of foot of his day, and he had seen much service under the Sforza; but his experience could avail him little here.
On the 28th Cesare opened the attack, training his guns upon the citadel; but it was not until a week later that, having found a weak spot in the walls on the side commanding the town, he opened a breach through which his men were able to force a passage, and so possess themselves of a half-moon.

Seeing the enemy practically within his outworks, and being himself severely wounded in the head, Naldo accounted it time to parley.

He begged a three-days' armistice, pledging himself to surrender at the end of that time should he not receive reinforcements in the meanwhile; and to this arrangement the duke consented.
The good faith of Naldo has been questioned, and it has been suggested that his asking for three days' grace was no better than a cloak to cover his treacherous sale of the fortress to the besieger.

It seems, however, to be no more than one of those lightly-uttered, irresponsible utterances with which the chronicles of the time abound, for Naldo had left his wife and children at Forli in the hands of the Countess, as hostages for his good faith, and this renders improbable the unsupported story of his baseness.
On December 7, no reinforcements having reached him, Naldo made formal surrender of the citadel, safe-conduct having been granted to his garrison.
A week later there arrived at Imola Cesare's cousin, the Cardinal Giovanni Borgia, whom the Pope had constituted legate in Bologna and the Romagna in place of the Cardinal Ascanio Sforza, and whom he had sent to support Cesare's operations with ecclesiastical authority.

Cardinal Giovanni, as the Pope's representative, received in the Church of San Domenico the oath of fealty of the city to the Holy See.