Andrew Manson’s struggle to hold on to his youthful idealism—a battle that he often loses—is at the core of The Citadel. A. J. Cronin suggests that pure idealism is the province of the young, and that it is ultimately unsustainable. As soon as he begins working, Andrew must confront the limits of putting his ideals into practice when he clashes with Mrs. Page about appropriate pay for his work. His efforts to pursue potentially beneficial research are often rebuffed and ridiculed. The London episode of his life immerses him in the guilty pleasures of material wealth, which are endorsed by Hampton, but ultimately proves incompatible with his character. Christine, in contrast, represents the pure idealism that cannot hold up in the long run. Although her death is an accident, having her life cut short symbolizes that idealism is incompatible with continued functioning in society.

Manson is presented as a fundamentally principled man whose quest for a satisfying professional life is deeply rooted in his strong morals. Although he is tempted by the apparent ease of working with rich people, he constantly questions the ethics of apparently unnecessary treatments. While others may argue the practical necessity of cutting corners, Cronin clearly rejects those views as hypocritical and even threatening to the medical profession. In Aberalaw, a committee of uneducated laymen challenges the value of Manson’s research, but they are motivated by concern for quality care. Freddy Hampton, who benefits financially and socially from promoting the provision of frivolous services, represents the moral bankruptcy of physicians who sell out.

One of Manson’s dilemmas is how to integrate the research aspects of medicine with the practical needs of his patients. Through his initial post in a town of coal miners, Manson becomes aware of the need for sustained research into the effects of this work on the men’s lungs. His attempts to engage in this research are unsuccessful, largely due to popular opposition. He must also admit, however, that his own priorities are for patient treatment. After his negative experience when his published article has almost no impact, Andrew realizes that each doctor must match their talents to the realm that suits them best.