If you read the entire story, you’ll find that the Demon King (who, by the way, is called “she” a whole bunch of times througout most of the story), never has any intention or expectation of taking the fortress. That’s probably the only “military secret” that really matters. And she doesn’t sell that secret.
But the Summoned Heroine doesn’t know that until the end, and it’s stated that she specifically set up the market to “help them anticipate and counter the Demon King’s next moves”.
If you read the entire story, you’ll find that the Demon King (who, by the way, is called “she” a whole bunch of times througout most of the story), never has any intention or expectation of taking the fortress. That’s probably the only “military secret” that really matters. And she doesn’t sell that secret.
The soldiers guarding the outer wall and the Citadel treasurer that pays their overtime wages would beg to differ.
But the Summoned Heroine doesn’t know that until the end, and it’s stated that she specifically set up the market to “help them anticipate and counter the Demon King’s next moves”.