Actually, this was because the formation was ancient and heavily depleted; otherwise, even if they found a weakness, they wouldn't have been able to break through in a short time.
After emerging, they found themselves standing in the exact same spot as before, but the city gates and walls had vanished, replaced by a black barrier.
This barrier was much sturdier than the previous one; a combined strike from everyone didn't even make it tremble.
Chapter 34