In the beginning of April 1547, he collected a considerable body of men and began to make approaches to the castle of Montobbio. To prevent the introduction of troops and supplies into the fortress he ordered Lamba Doria, Bernardo Lomellini and Gabriele Moneglia to seize the passes of the Apennines and keep close guard on the frontier. Gonzaga rendered valuable aid in these operations. He sent captain Oriola with a company of Spanish infantry to Torriglia with orders to assist the Genoese generals in divising means to approach Montobbio.
Though the roads were rocky and broken, Spinola brought up many guns by the way of the Gioghi and along the Scrivia, which is formed by the confluence of the Laccio and Pantemina under the heights of Montobbio. Flippo Doria, who had already acquired distinction in naval warfare, was assigned to the command of the artillery. Andrea required that Francesco and Domenico Doria should have command of a body of two thousand infantry. The commissaries of the Republic were Cristoforo Grimaldo Rosso, and Leonardo Cattaneo, with Domenico De Franchi, and Domenico Doria for substitutes.
Count Gerolamo did not lose courage at the sight of these formidable preparations to assail his stronghold, but applied himself diligently to increasing his means of resistance. He fortified the approaches, repaired the curtains, videttes and battlements, and added new bastions and other works of defence. He had already collected a large body of mercenaries and to cover Montobbio had garrisoned Cariseto and Varese. He asked vainly for the assistance of the French troops in Mirandola, and then turned his attention to negotiations with Pierluigi Farnese. This duke pretended loyalty to the empire, but he secretly furnished men and supplies, permitted his vassals in the mountains to enlist under the standards of Fieschi and instigated the people of Valnura and Trebbia to obstruct the passes in front of the imperial troops.
Gerolamo, knowing the worth of Verrina’s advice and courage and the intrepidity of Assereto and the band of heroes who had taken refuge in Marseilles, sent many messengers to urge them to share with him the peril and glory of the siege. These refugees had sent Ottobuono and Cornelio Fieschi to the court of France to plead their cause, and the king had received them with marks of favour and promised to restore their fallen fortunes. The assurances were reiterated frequently, but the French monarch took no steps to prove his sincerity. Verrina and Assereto grew weary of the tedious delay and accepted the invitation of Gerolamo without awaiting the return of the Fieschi, preferring the risk of battle to begging for aid which was always promised but never given. They crossed Piedmont and found means to enter Montobbio. Gerolamo received them with joy and committed the defence to their hands. Later, Ottobuono came to Mirandola and Verrina and Vicenzo Varese went there to aid him in urging the French commander to assist in the defence of the castle. They solicited in vain. This refusal of France to succour Gerolamo is a new proof that Gianluigi had not agreed to deliver Genoa into the hands of the French monarch. Francis was prodigal of promises, but he left the Fieschi to encounter the forces of the empire alone.
Spinola planted batteries on a height now called Costa Rotta near Granara, a village to the west of the castle; but though he bombarded the citadel for forty days he was not able to gain one inch of ground, while the fire of the fortress mowed down the flower of his troops and daily explosions of his own guns added to the loss of life. Besides, the inclemency of the season and incessant rains prevented the formation of lines of circumvallation. The besieged were greatly encouraged, and the soldiers of the Republic proportionately demoralized, by these circumstances. On the tenth of May the podestà of Recco was ordered to send to Montobbio as a reinforcement to the besiegers all the men of that commune between the ages of seventeen and sixty years.
On the contrary, Paolo Moneglia and Manfredo Centurione had obtained possession of Varese, with little loss of life, through the treachery of its commandant, Giulio Landi, who surrendered it hoping to obtain the investiture of the feud. But this success by no means compensated for the losses under the walls of Montobbio. The castle of Cariseto opposed a vigorous resistance to the troops of the Republic. The people of that feud destroyed the roads, constructed fortifications and closed up the passes which led to the place. Boniforte Garofolo succeeded at length in forcing a path across the rugged summits of the surrounding hills and stormed the out-lying defences. The attack began at dawn of the 14th of April. The besieged flocked to the parapets, loop-holes and barbicans, and with their musquetry and cannon held the assailants at bay. The battle lasted the entire day. On the morrow, the Genoese artillery shattered a large tower which fell burying a considerable part of the defenders under its ruins. This misfortune discouraged the rest and they offered to make a conditional surrender of the place. Garofolo demanded a surrender at discretion, and the garrison insisted upon security for their lives and property. Gian Francesco Niselli, a friend of Fieschi and Pierluigi Farnese, was by accident in the place at the time of the assault, and he, seeing the hopelessness of the defence, sent messengers to Count Paolo Scotti requesting him to obtain the permission of Farnese for the retreat of the garrison into the territory of Piacenza. The duke readily consented, and the peasants and soldiers effected their retreat in the following night. They lit up fires on the side of the place which the enemy held and retired over broken and difficult foot-paths through the mountains.
The duke had been deeply affected at the death of Gianluigi; but to avoid a rupture with the empire he had sent Ottavio Bajardi to Ferrante Gonzaga, offering his troops and even his own person to the imperial cause. But he at the same time contrived to have the Pope secure him immunity from imperial demands. He sent Agostino Landi, count of Compiano, to congratulate Doria on his escape from the perils which had overhung his house and sent back to him a great number of fugitive slaves, belonging to the Doria galleys, who had taken refuge in the mountains of Piacenza. He afterwards sent Salvatore Pocino to the emperor to deny charges of complicity with Gianluigi. The emperor knew all the facts and received the envoy with great coldness; but the duke’s son who was in the imperial service pleaded more successfully for his father.
Meanwhile, the large imperial army, which had been massed in Varese to support the siege of Montobbio, kept the duke in constant apprehension that it might be destined to punish him for his treachery. These fears were strengthened by the fact that Gonzaga had added to Vistarino and Oriola five other captains, Sebastiano Picenardi, Lodovico da Borgo, Pier Francesco Trecco, Osio Casale and Gianfrancesco Ali, with considerable bodies of troops and strict orders to levy new recruits in Monticello and Castelvetro, feuds of the duke. To provide for the danger, Farnese, who had Cornelio Fieschi under his protection, reorganized the army of twelve thousand infantry which he had collected in January at Cortemaggiore, sent commissaries to forbid enrolment of imperial troops in his feuds, fortified the castles in his jurisdiction, placed six hundred infantry at Borgo, a greater number at Bardi and ordered Francesco Clerici commanding at Compiano to be on the alert and in constant readiness for battle. Shortly after he instructed his commissioner in Venice to ask the consent of that Republic to his drawing eight thousand arquebuses from Brescia. He was allowed to draw only five thousand. These operations led to reciprocal suspicions, rancours and threats between Farnese and the imperial captains, and Gonzaga, to prevent an open outbreak, recalled Vistarino from Bobbio.
This measure relieved Farnese from his present peril and he resolved to take advantage of the siege of Montobbio to get possession, in advance of the imperial troops, of some feuds of the Fieschi. He seized Calestano, and then sent Gianantonio Torti with a strong force to occupy Valditaro. As the Fieschi had some imperial vassals in these feuds, Farnese informed Gonzaga that he wished to hold them for the interests and rights of the empire. He did not wait for an answer, but hurried his troops into the feuds. His designs upon Valditaro were thwarted by Scipione and Cornelio Fieschi, who threw themselves into it with about one thousand of their vassals and shut the gates in the faces of the Ducal forces. He called Scipione to himself in Piacenza and persuaded him that the forces of his family were too weak to contend with the empire. Scipione consented that the duke should occupy the castle in the interest of his family. He returned to his vassals and persuaded them to enlist in the service of Farnese, who sent his agent, doctor Giovanni Landemaria, to take possession in his name. The acts of the notary Bartolomeo Bosoni clearly prove these facts.
Gonzaga was enraged at this stratagem of Farnese; and in fact the occupation was of short duration. On the death of Farnese, Valditaro was created a principate by the emperor and passed to Agostino Landi whose ancestors had once held it. The inhabitants always retained their love for the Fieschi house, and remembered long the mild government of their old masters. They several times conspired to restore Scipione who was born among them. In 1552, Gonzaga, incensed at these movements, instigated Landi to dismantle the forts and towers lest they should afford a place of refuge for the Fieschi.