I thought Ousen’s goal was to stop the Atsuyou Army, not take Atsuyou itself?
Although I suppose that if the Atsuyou Army under Riboku is heading towards Gyou then the defences of Atsuyou itself will be weaker than usual.
Still, Atsuyou must have some formidable defences considering it is of strategically important value and the fact that Ousen called it a “Great City”. There is also a high possibility that Riboku upgraded Atsuyou’s defences just like he did with Gyou. Ko Chou might be there too (seriously, where is that bloke? Lol).
Sieging Kantan is a seriously bad idea. The Ousen Army is damaged, spent and low on supplies.
Meanwhile, Kantan has an army 100,000 elites, possibly even with Ko Chou in command.
It being the capital would also mean that it likely has some incredibly sturdy defences.