The Citadel follows the life of Andrew Manson, a young and idealistic Scottish doctor, as he navigates the challenges of practising 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.

A groundbreaking novel of its time and a National Book Award winner, The Citadel has been adapted into several successful film, radio, and television productions around the world, including the Oscar-nominated 1938 film starring Ralph Donat, Rosalind Russell, Ralph Richardson, and Rex Harrison.