Xu Qinghuan folded the sheet of rice paper covered in trivialities from the capital and pressed it beneath the inkstone.
"Qingque, bring the lantern."
Xu Qinghuan turned and walked straight for the door. "To the General's Mansion main hall."
Qingque hurriedly picked up the lantern and followed with quick, short steps.
As the mistress and maid passed the eastern corridor, they ran head-on into a squad of armored night patrolmen.
The corporal leading them, seeing it was the Imperial Envoy, immediately halted and saluted with clasped fists.
"Your Excellency!" the corporal said respectfully.
Xu Qinghuan paused.
"Speak."
"General Xu Chengguang, two hours ago, mustered the full three thousand cavalry of the Pacifying Qiang Army under his command. They rode out through the south gate overnight, heading back to reinforce West Route Prefecture."
"He specially asked me to relay this to you."
Xu Qinghuan's gaze turned cold and sharp.
The city walls of West Route Prefecture had already been breached in multiple places by Ashina Gudu's trebuchets.
Under that kind of ten-thousand-catty siege engine, with crumbling ruins and broken battlements, what defensible terrain was even left?
For Xu Chengguang to charge his troops straight in like that was like a sheep walking into a tiger's den.
Those three thousand Pacifying Qiang cavalry were likely facing an uncertain fate.
But for now, Zhenbei Pass itself was barely holding, and this quagmire was only sinking deeper.
She forced that concern down.
Xu Chengguang had his own fate to meet; she had her own mess to clean up.
"Understood." Xu Qinghuan resumed walking. "Continue your patrol. Do not grow lax."
Inside the General's Mansion main hall, candlelight blazed bright.
Tie Lanshan was frowning at the sheepskin map spread across the desk, his brow furrowed into a deep crease.
He heard light footsteps and saw Xu Qinghuan returning in the middle of the night.
"Magistrate Xu?" Tie Lanshan straightened up, looking puzzled. "Has the situation at West Route Prefecture taken another turn?"
Xu Qinghuan strode quickly to the long desk and pressed both hands on its edge.
"Marshal Tie, there's a way to break this dead-end game."
Hearing this, the towering old general gave a violent start.
He too braced his hands on the edge of the desk and pressed forward urgently.
"Truly? Where's the key to the breakthrough?"
"It's not outside—it's inside." Xu Qinghuan extended a slender finger and jabbed hard at the very center of Zhenbei City on the map. "Right beneath our feet."
Tie Lanshan followed where her finger landed and grew more confused by the moment.
"Beneath our feet?"
"Just now I received a letter from my eldest brother in the capital." Xu Qinghuan wore the lightness of one who had narrowly escaped disaster.
"The letter complained of a rat infestation in the family's old residence. They had people dig into the ground and only then discovered the foundations had been hollowed out by the rats—an entire network of interconnecting tunnels."
Tie Lanshan listened with growing impatience and slapped the side of the desk.
"Magistrate Xu, the fire is right at our eyebrows! What time is this to talk about letters and rats? Chen Changfeng is trying to drive us to a dead end at West Route Prefecture!"
Xu Qinghuan couldn't help but smile at the marshal's expression.
"Marshal Tie, that feint of Chen Changfeng's—drawing you to guard the gates. What were you defending against?"
"Naturally, guarding against the bastards breaking in from outside."
"And what if the bastards don't need to break through the gates at all?"
"They don't break through the gates? How do they enter the city? Sprout wings and fly in?"
Xu Qinghuan stopped beating around the bush.
"Dig—a—tunnel."
At those words, a thunderbolt seemed to strike Tie Lanshan's mind.
He stood frozen on the spot.
Tie Lanshan was no fool; realization dawned instantly. Could it be... from inside the city... digging outward?
Such a reversal of the usual method!
But if it were those hidden agents who had been lurking in Zhenbei Pass for who knows how long—
If they had used the cover of civilian homes, grain shops, even military barracks, digging earth day and night without rest, then mixing the soil into the daily dung carts and firewood carts to haul it away and bury it...
Cold sweat beaded on Tie Lanshan's forehead.
He slapped his own thigh in frustration, a single heavy blow.
"Fool! This old fool!"
The veteran marshal who had spent half his life guarding the frontier had red-rimmed eyes from suppressed anguish.
"I, Tie Lanshan, have been in the north for half my life, and all I ever watched was how thick the walls were, how stout the gates were!"
"I defended against warhorses, against heavy crossbows, but I never once looked down at the very ground I was standing on!"
Xu Qinghuan stood quietly, saying nothing.
An old general's self-reproach needed no words from others.
Tie Lanshan took several rough breaths in his anger, pacing back and forth.
"But the ground beneath Zhenbei Pass is no ordinary ground."
He walked to the brick wall on the left side of the hall and pointed at a topographical map hanging there.
"When the late emperor chose this site to build the city, what he valued was the granite stratum underneath."
"That rock layer is so hard that even chisels and axes struggle to leave a mark. When they laid the city's foundations, they wore out who knows how many iron picks. An enormous undertaking."
Tie Lanshan's brow knitted heavily.
"Even when the city tried to dig wells for water, the artisans had to take probing rods and survey again and again—two full weeks of effort before they barely found a few water veins in the eastern and southern districts."
"To carve a tunnel that could pass troops and horses under all that... more impossible than climbing to heaven."
Xu Qinghuan picked up the thread.
"Marshal Tie just mentioned it yourself—searching for water veins."
"Those rock layers underground aren't necessarily one solid, seamless sheet of iron everywhere."
"Where water can flow, there must be cracks in the stratum or softer, weaker spots."
Xu Qinghuan looked Tie Lanshan straight in the eye.
"Chen Changfeng must have been laying this plan for no less than five years. As long as his hidden agents worked along those weak rock seams, spending five years at it—dripping water wears through stone."
When Tie Lanshan heard this, every trace of color drained from his face.
Digging along the weak spots...
Five years.
Even if they'd been clawing with bare hands, they would have gouged out a road to hell leading straight out of the city.
His mind churned like a storm, and suddenly an old military treatise he had once read surfaced in his memory.
"Siege by tunneling." Tie Lanshan muttered aloud.
"During the Three Kingdoms, Yuan Shao used that method against Gongsun Zan; there are precedents from the Tang Dynasty as well. How could I have forgotten that one entirely?"
He slammed his fist against a red-lacquered pillar, shaking dust from the beams above.
"I, Tie Lanshan, pride myself on knowing warfare. These last few years I drilled the crossbows and city defenses into an iron bucket, yet I threw this deadly underhanded trick clean out of my mind. I nearly brought this city to ruin!"
As soon as he finished speaking, Tie Lanshan suddenly stopped moving, then threw his head back and burst into laughter.
The laughter carried the reckless relief of someone who had just escaped death by a hair, mixed with the lingering aftershock of belated terror.
"Ha ha ha—Well played, Chen Changfeng! What a vicious scheme!"
"So close! This close! This old man nearly fell to the very city defenses I was most proud of!"
If this plot hadn't been uncovered tonight—
He would still be stubbornly guarding the gates.
And when Helian Tiefutu's ironclad cavalry came swarming up out of the tunnels into the barbican, into the barracks, Zhenbei Pass would truly have become nothing more than a slaughterhouse awaiting the blade.
Tie Lanshan's laughter faded. He turned to look at Xu Qinghuan, his eyes full of respect.
"With strategy like yours, Magistrate Xu, I am completely convinced."
"Chen Changfeng's chain of absolute ruin fooled tens of thousands of eyes in this city, but it couldn't escape your sharp sight. How did you manage to deduce the tunneling method from just a trace of Chen Changfeng's machinations?"
Xu Qinghuan waved a hand modestly.
"Marshal Tie flatters me too much."
"Of troop formations and military stratagems, my understanding is honestly lacking."
She pointed out the door at the pitch-black sky beyond.
"This is all thanks to my eldest brother back in the capital."
"If it weren't for the rambling bit in his letter home about a rat infestation and how the ground beneath had been hollowed out into a connected network of tunnels, I would never have guessed that Chen Changfeng would resort to such a devious trick."
Tie Lanshan was stunned for a moment, then burst into laughter, clapping his hands.
"Eldest Young Master Xu!"
"That eldest son of the Xu family at the Ministry of Rites is truly a blessing to our borderlands!"
Tie Lanshan's voice boomed.
"Here he sits in the capital, a thousand li away, and a casual letter about household trifles manages to pierce through the most perilous deadlock of the northern frontier."
"Once we repel the enemy in this battle, I, Tie, will personally petition the court to commend the young master for his contribution!"
Xu Qinghuan did not take the bait of such flattery. She set the topic aside and pulled the conversation back on track.
"Marshal Tie."
Xu Qinghuan's expression was as grave as still water.
"Since Chen Changfeng sent Zhao Cheng out to cause a diversion, it means that tunnel has already been dug to the final stretch—just one thin layer of wall away from breaking out beyond the city."
"The priority now is to find out exactly where this rat, which has been hiding in Zhenbei Pass for five years, has made its nest within the city."
Hearing this, the smile on Tie Lanshan's face vanished entirely.
The old marshal's gaze turned cold and sharp. He clasped his hands behind his back.
"We must not breathe a word of this."
There are spies everywhere in the city. If we launch a full-scale search for the tunnel, we'll alert the snake.
Tie Lanshan's eyes burned like torches.
"Once those insiders catch wind that our suspicions are roused, they'll panic and, caring nothing for the consequences, break through the remaining stretch of tunnel ahead of schedule to let the barbarians in from outside—and then the whole city's populace will pay the price."
Xu Qinghuan nodded in agreement.
An old general was, after all, an old general.
Even in a moment of excitement, he could steady his footing at once.
"What plan does Marshal Tie have in mind? To expose the tunnel's mouth without disturbing the culprits will be no easy task."
Tie Lanshan walked behind the long desk and, from a towering stack of official documents, fished out a worn yellow register.
It was the complete census of all residential homes, grain shops, and storehouses in Zhenbei Pass.
"That tunnel was dug underground for five years. The volume of excavated earth must be enormous."
"An ordinary family's small courtyard couldn't possibly hide that much useless dirt."
Tie Lanshan flipped open the register, dragging his calloused finger heavily across the pages.
"We need a place that, on the surface, already sees constant movement of goods in and out—or a sprawling site big enough to absorb a mountain of waste soil."
"A large merchant's warehouse? Or a long-abandoned, overgrown estate?"
Xu Qinghuan stepped forward half a pace.
"And that's not all. These men digging the tunnel would make a considerable racket and would need to cover up the noise. Is there any trade in this city that runs day and night with clamor?"
Tie Lanshan's thick brows furrowed as he quickly ran through everything in his mind.
"A blacksmith's forge? A carpenter's workshop? Or else—"
Both of their gazes landed at once on a single page of the register.
"The mule-and-horse market in the southwest corner of Zhenbei Pass!" Tie Lanshan slammed his palm onto the volume.
Xu Qinghuan picked up the thread. "The livestock market has animals braying and neighing at all hours—sheer constant noise. And they regularly cart out manure and fodder scrap, mixed with soil, out of the city. No one would ever raise an eyebrow."
"Over five years, they could easily disguise themselves as laborers hauling dung and refuse, driving the dirt from the tunnel out through the gates in broad daylight, one cart after another."
Tie Lanshan was deeply persuaded.
"Magistrate Xu speaks sensibly. That mule-and-horse market is a den of all sorts—a melting pot of itinerant porters and camp followers from every caravan. It's the perfect place to hide filth and harbor villains."
"And more than that, the southwestern corner sits against the base of the old city wall, where the stone was fractured once by an earthquake a century ago—the weakest point of them all."
And with that, the loop closed end to end.
The mystery was torn open, piece by piece.
Tie Lanshan's eyes blazed with fierce battle-readiness.
