As a temporal prince who constructed stout fortresses in the Papal States, he encouraged the Venetians to attack Ferrara, which he wished to obtain for another nephew. Ercole I d'Este, Duke of Ferrara, was allied with the Sforzas of Milan, the Medicis of Florence along with the King of Naples, normally a hereditary ally and champion of the papacy. The angered Italian princes allied to force Sixtus IV to make peace to his great annoyance. For refusing to desist from the very hostilities that he himself had instigated and for being a dangerous rival to Della Rovere dynastic ambitions in the Marche, Sixtus placed Venice under interdict in 1483. He also lined the coffers of the state by unscrupulously selling high offices and privileges.