The Citadel follows the life of Andrew Manson, a young and idealistic Scottish doctor, as he navigates the challenges of practicing medicine across interwar Wales and England. Based on Cronin's own experiences as a physician, The Citadel boldly confronts traditional medical ethics and has been noted as one of the inspirations for the formation of the National Health Service.

This groundbreaking novel was a distinguished achievement of its time. It presents an honest and moving study of a young doctor, showcasing great literary taste and skill.

The Citadel has been adapted into several successful film, radio, and television productions around the world, including the Oscar-nominated 1938 film.