He was a favourite of Charles the Dauphin (later Charles VII), whom he saved by taking him out of Paris to Melun at the time of the invasion of Paris by the Burgundians led by Jean de Villiers de l'Isle-Adam during the night of 28–29 May 1418. With Jean Louvet, another of Charles VII's favourites, he was one of the main instigators of the assassination of John the Fearless by some Armagnac men-at-arms during his meeting with Charles on the bridge at Montereau on 10 September 1419.
From 1425, his influence waned as Arthur de Richemont's waxed. Also, in 1429, he used all his effort to convince the Dauphin to receive and welcome Joan of Arc – in effect, several of Charles VII's counsellors supported the principle of a rapprochement with Burgundy in order to present a united front against England, which could not have been achieved without du Chastel's efforts.
Histoire des ducs de Bourgogne de la maison de Valois, 1364–1477 de Prosper Brugière baron de Barante- 1782-1866, conservé à la Bibliothèque nationale de France (BnF N087463). For a detailed account of the "affaire du pont de Montereau" (Extracts from the document)