Fort Nanko is a defensive line that extents all along the border with Chu. This, in itself, clearly demonstrated the "fear" that Qin held towards the superpower of Chu but with such sturdy defenses erected, Chu itself could not launch an attack lightly either. Hence, it was accepted that Chu would never attack fort Nanko because doing so would result in a big war.
This led to lax security and left them unprepared for the shock of invasion.
Fort Nanko extents all along the Qin border with Chu. There are high walls which stretch the length of the Qin-Chu border and was later used as a model for the Great Wall of China.