The mention of Château d’Avalon is found in the romance Les Romans de la Table Ronde, a collection of medieval French prose romances. Specifically, it is mentioned in Chréten de Troyes’s Lancelot ou le Chevalier de la Charrette.
In this romance, Château d’Avalon is described as the dwelling place of the evil knight Meleagant (Meliagrant), who abducts Queen Guenevere, the wife of King Arthur. The abduction of the Queen becomes the central plot point of the story, and Lancelot embarks on a perilous quest to rescue her from Meleagant’s clutches.
Château d’Avalon is depicted as a mysterious and distant place, and it serves as the setting for some of the crucial events in the romance. In some versions of the tale, Château d’Avalon is associated with the legendary island of Avalon, known as the place of rest and healing where King Arthur was taken after his death.
Source
Lancelot, or Le Chevalier de la Charrete | Chrétien de Troyes, late 12th century