Blind Bei had invested considerable effort into the design of Green Willow Fortress. However, its defensive capabilities weren't his utmost priority, as he never envisioned a day he'd be forced to defend it while awaiting reinforcements. He reasoned that, not only was it unlikely for Qian State to launch an offensive, but even if enemy forces did attack and prove too strong, he could simply flee with his subordinates and belongings.
Nevertheless, Blind Bei had meticulously planned the finer details, including this secret tunnel. He had informed Zheng Fan about it and had even personally guided him through it once.
The room above the tunnel had originally belonged to A Ming. However, when Blind Bei departed for Qian State, he had A Ming exchange rooms with Shatuo Queshi. This led to the blunder where Zheng Fan, upon his return, walked into the wrong room and inadvertently spoke his "inner thoughts" aloud.
