As the primary theme of I Capture the Castle, the concept of authorship and its creative, social, and financial obligations is centered on the characters of Cassandra and James Mortmain. Cassandra and her father are trying to understand their personal authorship in relation to each other and the lives of their family, with the novel’s conclusion showing both embracing the role that authorship plays in their lives. Furthermore, the introduction of the Cotton family to their social circle—and the Cottons’ love for literary arts—motivates Cassandra and her father to understand their personal version of authorship.

As the novel is told through Cassandra’s journal entries, the novel itself embodies her search for authorship and her eventual acceptance of what authorship means to her. She begins with a desire to “capture” the castle through description. More than that, however, Cassandra uses journaling “partly to practise my newly acquired speed-writing and partly to teach myself how to write a novel—I intend to capture all our characters and put in conversations” (4). In this way, Cassandra’s journal chronologizes the discovery of her authorship, as she learns that her journal allows her to write the kind of novel she wants to write: “Now that life has become so much more exciting I think of this journal as a story I am telling” (131).