Cardinal de Castelnau served as Ambassador of King Louis XII of France to Pope Julius II beginning in 1507. On 2 May 1509, the Cardinal was elevated to the rank of Cardinal Priest and assigned the "titulus" of San Stefano al Monte Celio. Unfortunately, his diplomatic skills were insufficient to deal with the volcanic temper of Pope Julius II. During the War of the League of Cambrai he was too forthcoming in his exchanges with the Pope on behalf of France and its King, and on 29 June 1510, when he demanded his passport, the Pope, in a fit of anger by no means becoming his high office, sent Cardinal de Castelnau instead to detention in the Castel S. Angelo. On the petition of the College of Cardinals he was released from the Castel S. Angelo, but was kept under arrest in the Vatican Palace, but the Pope indicated that he would not be released until the prelates who had been taken prisoner in the capture of Bologna by Louis XII were released. Shortly thereafter, after swearing not to adhere to the Conciliabulum of Pisa, he was given his passport and allowed to return to France.