Using the same logic, one could say that entering the underground should only require passing through one guard, because that's how it works in regular passages.
I think that in 50%+ of "regular" passages, there are two or more guards. The passage can be wide and also divided into two or three sections, each with its own guard, even though essentially there is only one passage.
"Сущность войны — обман. Искусный должен изображать неумелость. При готовности атаковать демонстрируй подчинение. Когда ты близок — кажись далёким, но когда ты очень далеко — притворись, будто ты рядом."