The definition of a "citadel" is a type of fortress almost impenetrable to attack. It stands alone "typically on high ground" as a form of protection. In many regards, Andrew Manson's idealism is a citadel for him. Andrew's faith in medicine is a citadel. At some points in the narrative, this citadel comes under attack through materialism. However, Andrew's idealism is what emerges throughout the narrative, fortifying the citadel of why he does what he does. When Andrew speaks in front of the inquiry, the citadel of his idealism is evident:
What's happening now? Many doctors make fortunes from their patients by giving them medicines and expensive treatments that are useless! It isn't right! It isn't honest! I have made many mistakes myself, and I am sorry about them. But I made no mistake about Stillman, and I was right to turn to him. I ask you to look at Mary Boland. When she went to Stillman, she was dangerously ill. Now she's cured. Judge my actions by that!
Manson clearly recognizes that there are problems within the medical system. He understands that the problems which exist challenge purpose of medicine. He also comprehends his own complicity in allowing these forces to penetrate his citadel of pure medicine. However, it is clear that Andrew takes refuge in the sanctuary of his idealism as a way to repel the forces that threaten medical science. When he pleads with the inquiry to "judge" him on what he has done and how he has "cured" people, it is clear that Andrew's idealism is a citadel in which shelter and protection can be taken from the material forces pose significant threat.