The word Citadel can mean a 'bulwark' and the term has often been used to describe the 'Health Services' provided by UK Government / the Hospitals therein (latterly since 1948, a term used for the NHS)
A J Cronin's background - he was a doctor-
...... After WWI he set up a practice in a mining area of South Wales, and in 1924, he was appointed Medical Inspector of Mines. He drew on his experiences researching the deleterious effects of the mining industry on the workers' health for his novel The Citadel, set in Wales ......
Many believe that Cronin's title selection of The Citadel for this work published first in 1937 was in support of the content in his book .... and, was intended to link the book to the poor provision of healthcare in the UK.
The Citadel was seen more strongly, then, as a 'thinly veiled' critique of health services of the time (especially the lack of provision for mineworkers)
{The Citadel is considered by many to have contributed to the establishment of the National Health Service in Great Britain by exposing the injustice and incompetence of medical practice at the time.}