Answer:
- military
Step-by-step explanation:
"Citadel of learning" as used in the passage refers to an environment that is - military.
From the passage, we see that the speaker talks about the blessedness of a man. The speaker talks about the blessedness of geniuses who know that egomania is not a duty.
The speaker reveals that in the citadel of learning, there is a fort that armors them well. It's an environment that is characterized with diversity, controversy and tolerance.