I guess a xingtai army is a standing army with discretionary powers.

Isn’t Prince Yu supposed to be confined in his house? Why is he busting out to visit the prison all the time?

Reinforcing the narrative of Banruo's fallen kingdom in the drama not only enriches Prince Yu, it also makes those loose threads feel more meaningfully integrated with the whole story and better-illustrates the thematic parallels between her mission and Changsu's.