So far, I agree with you entirely.

Tell me, now, how the thing is to be accomplished." And there was sarcasm in his voice.
"What difficulties does it present ?" inquired Gian Maria.
"No difficulties," was the ironical reply.

"She has shut herself up in the stoutest castle in Italy, and tells me that she will not come forth until I promise her freedom of choice in the matter of marriage.
Clearly, there are no difficulties attached to her being brought back." Gian Maria showed his teeth.
"Do you give me leave to go about it in my own way ?" he asked.
"Not only do I give you leave, but I'll render you all the assistance in my power, if you can devise a means for luring her from Roccaleone." "I hesitate no longer.

Your niece, Lord Duke, is a rebel, and as a rebel is she to be treated.

She has garrisoned a castle, and hurled defiance at the ruler of the land.