Count Gaston's meeting with Charlotte was no secret.
In fact, the mobilization of an army of this scale could not be concealed.
Before Charlotte had even left the Northern Lands, the nobles there knew she was going to meet with the Commander of the standing army in the Tulip Count Domain. Similarly, in the Tulip Count Domain, there were officers loyal to the Bold family who remained behind.
Nevertheless, Charlotte hadn't intended to keep the meeting a secret.
Moreover, after returning to North Port, she publicly announced the outcome of the meeting—that she would form an Allied Forces with Count Gaston, who was stationed in the Tulip Domain, to march north and eliminate the rebels of Castel.
The Tulip Count Domain did the same.
Upon returning to the fortress, Count Gaston swiftly organized his military resources and declared that he would assist Count Castel in quelling the rebellion to the north.