The fortress was radically transformed by Innocent III and Urban V. The latter chose it as his summer residence during the years of his stay in Italy: from 1367 to 1370. During the period of the Avignonese captivity, it was the centre of political affairs for the entire Patrimony of San Pietro in Tuscia, the term used to indicate the current province of Viterbo. It played a crucial role mainly thanks to Cardinal Egidio Albornoz. Towards the end of the 15th century, Alexander IV initiated a programme of military adaptation of the papal fortresses, and Antonio da Sangallo the Elder drew up a project for the renovation of the Fortress of Montefiascone in order to adapt it to the new war requirements that had arisen with the use of gunpowder and cannons. In the very first years of the 16th century, Pope Julius II ordered the continuation of the restoration and Leo X then entrusted the direction of the building to Antonio da Sangallo the Younger. The beginning of the monument's decline is attributed to Paul III Farnese, who moved the cannons from the Rocca to the Fortress he had erected in Perugia and the seat of the Rector of the Patrimony to Viterbo. At the end of the 17th century, the Fortress was ceded in usufruct to Cardinal Barbarigo, who took material from it for the construction of the Seminary and used the surviving structures as a warehouse and printing works.