In this realistic war game, only I understand the course of the battle.

Spring of the twenty-second year of the Yongle reign, Beijing.

When Shen Du returned to Beijing from Jiangxi, he was caught in a sudden cold snap. Snowflakes fell thickly from the leaden sky, landing on the glazed tile roof of Deshengmen and the newly sprouting willow branches along the moat, suppressing the budding spring. As he rode his horse and gazed at the majestic Deshengmen gate in the distance, he suddenly remembered that it had been three years since he last passed through this gate. In those three years, he had traveled to Guangxi and Jiangxi, repairing fortifications, building dikes, and comforting refugees, traversing tens of thousands of miles. Now he was back. Beijing was still Beijing, Deshengmen was still Deshengmen, but the people standing at the gate were no longer the same.

Old Zhao, riding on the donkey's back, hunched his shoulders and tucked his pipe deeper into his cotton-padded coat. He'd been bouncing along the donkey for over half a month, the conditions at the post stations along the way from Jiangxi to Beijing were limited, and he'd barely had time to wash with hot water. He glanced towards Deshengmen from the donkey's back, pulled out his pipe, and exhaled a puff of white breath: "Old Li, do you think anyone in the city still remembers us after all these three years?"

"I remember." Shen Du gently spurred the horse's belly, making it walk a little faster.

The Hall of Supreme Harmony was still the same Hall of Supreme Harmony, but the crabapple tree in front of it had grown thicker, its buds still covered in snow. Zhu Di sat behind his desk, wearing a worn-out dark blue casual robe, the sleeves rolled up to his elbows, revealing the old arrow scar on his forearm from Dongchang. He was reviewing memorials when he heard the announcement from outside the hall, so he put down his pen and looked up. Shen Du knelt on one knee to pay his respects, but Zhu Di did not immediately tell him to rise. He stood up from behind his desk, walked to Shen Du, and looked down at him for a long time. The young man who had survived in the reeds of the Baigou River had now grayed at the temples, deep wrinkles around his eyes, and a slight stiffness in his left leg when he stood straight, even when kneeling. From the first year of the Yongle reign to the present, he had painted the vast landscape of the Ming Dynasty, built countless city walls and fortifications, and pacified countless refugees. But Zhu Di himself was no longer the prince who braved the snow to direct the crossing of the river on the embankment of Jiahe—his hair had turned completely white, his back was still very straight, but when he stood up and walked to the palace gate, his hand would subconsciously touch the door frame.

"I'm getting old," Zhu Di said. He spoke the word very softly, as if he were talking about himself, or perhaps about Shen Du.

"Your Majesty is in good health, why do you say such a thing?" Shen Du said.

Zhu Di smiled, walked back behind his desk, sat down, and pushed a memorial on the desk aside. "I am already sixty-five. Since the Jingnan Rebellion, how many times have I gone beyond the Great Wall? Five times, a full five times. I personally led cavalry to pursue the Tatars, from the Luqu River to the Onon River, and from the Onon River to the Tula River. I scattered the Tatars, subdued the Oirats, and drove Arughtai away. I have fought on horseback my whole life, but now I am old, and so are my horses. But the north is not yet at peace. Although Arughtai has fled, the remnants of the Tatars are still wandering the grasslands, and who knows when they will return."

He paused, looking at Shen Du. "I summoned you here because I want you to make a trip to the northern frontier on my behalf. From Juyong Pass to Datong, from Datong to the Hetao region, from the Hetao region to Xuanfu, go and retrace all the passes and garrisons you surveyed in the early years of the Yongle reign. See if the city walls have collapsed, if the farmland has been abandoned, and if the garrison has become lax. In Jiangxi, you recruited refugees and distributed land to them to cultivate. In Xuanfu and Datong, I have also allowed military households to cultivate a lot of farmland over the years, but I don't know if they are cultivating enough to feed themselves, or if the city walls they have built are strong enough. Go and take a look, and come back to tell me."

Shen Du kowtowed. "Your subject obeys the decree." He raised his head and saw an indescribable look in Zhu Di's eyes—not the look of an emperor looking at a subject, but the look of an old general who had fought most of his life entrusting his affairs to another old soldier.

After leaving Fengtian Hall, Shen Du didn't return directly to the post station. He walked west along Chang'an Street, reaching the gray-tiled, brick-and-mortar neighborhood in the western part of the city. This was the resettlement area for the families of soldiers who died in the Jingnan Rebellion. Each household had a wooden plaque nailed to its door, bearing the name of the fallen soldier and their garrison. The wood of the plaques had weathered over a decade of wind and rain, and many were cracked, but the ink on them remained clear. Next to some plaques hung newly replaced peachwood charms, inscribed with auspicious words, expressing the residents' simple wishes for their families' safety. Shen Du started from the first household in the first row, looking at each one. Zhang Tiezhu, Liu Shitou, Zhao Da—these were names he had personally written in the military merit register after the Battle of Baigou River. He stopped in front of one of the households. The door read, "Zheng Biao, Left Guard of Yanshan, Pocheng Camp, died in battle at Dongchang."

Zheng Biao. The young man who seized the Southern Army captain's sword at the gates of Dezhou, the one with the words "Zhenhuai" engraved on the hilt, the one who polished the sword until midnight. Shen Du remembered the words he shouted as he charged back at the Dongchang moat during his final assault—"Master Li, I'll go first!" Later, he was shot through the throat by a poisoned crossbow bolt and fell beside the moat, Zhang Yu's body not far from him. Shen Du stood before that door, silent for a long time. He reached out and touched the inscription on the wooden plaque, took out a bag of silver from his pocket, hung it on the door knocker, took two steps back, and bowed deeply. Then he turned to the next house.

Old Zhao squatted at the alley entrance, turning his pipe over and over in his hands, head bowed and silent. He had fought for several years during the Jingnan Rebellion, from Baigou River to Nanjing, from Dezhou to Lingbi. He had always thought his most painful moment was seeing Zheng Biao being dragged away from the pine forest by the Southern army at Qimei Mountain. But now, standing before these wooden plaques, he suddenly felt that night at Qimei Mountain was nothing. The light in these homes had shone under these plaques for over a decade.

After leaving the neighborhood, Shen Du went to the Ministry of War. The Ministry was bustling with people; several young clerks were jogging down the corridor carrying files. One of them bumped into a doorframe, scattering the files all over the floor. Shen Du bent down to help him pick them up. The young man looked up and saw the clouded leopard on his insignia, quickly bowing respectfully, his words stammering with nervousness. Shen Du patted him on the shoulder: "Slow down, no rush." ​​When he entered the duty room of the Ministry of War's Department of Military Affairs, a young clerk was hunched over a desk drawing a map. He was drawing a map of the distribution of garrisons in the Hetao region; the charcoal pencil scratched across the paper, some lines crooked and disjointed. Shen Du stood behind him for a while and then said, "The old course of the Yellow River in the Hetao region was changed once in the seventh year of Yongle, so the location of the ferry crossing marked on the old map is no longer accurate. Move the ferry crossing fifteen li to the east, where the riverbed is narrower and more stable, making it suitable for building a pontoon bridge. In addition, there are no beacon towers marked between the garrisons on the west side of the Hetao region—without beacon towers, these garrisons are blind and deaf."

The young officer quickly bowed, his fingers still stained with charcoal ash. Shen Du took the charcoal from him and marked the locations of the beacon towers on the map, then briefly sketched the regular patrol routes between several garrisons in the margins. The young man nodded repeatedly. Shen Du handed the pen back to him: "Drawing a map isn't just about drawing mountains and rivers; you have to clearly mark every pass, every watchtower, and every passable ravine. Maps are for fighting, not for hanging on the wall for decoration."

It was already dark when he left the Ministry of War. The streetlights on Chang'an Avenue were just coming on, and the newly built glazed tile roofs gleamed a dark blue in the twilight. From a teahouse on the street came the voice of a storyteller, recounting Zhu Neng's victory at Jiahe. Shen Du stood by the street listening for a while, thinking of Zhu Neng—who had died on his way back from Jiaozhi the year he returned to Beijing. This old brother who had fought alongside him from Dezhou all the way to Lingbi had ultimately failed to return to the Beijing he had helped build. He stood there for a moment, then continued walking. Passing the Ministry of Revenue, he saw lights still on inside. Su Wanqing was on night duty. He hesitated at the entrance, not going in, but instead having the gatekeeper deliver a package of medicine he had brought from Jiangxi and a letter. The letter contained only one line—"The plaster has been received. My leg injury is much better, don't worry."

In the spring of the twenty-second year of the Yongle reign, Shen Du set off from Beijing once again, retracing the route he had taken during his first survey twenty years earlier, retracing the entire northern frontier. Zhao Laoliu followed behind on his donkey, its back laden with new surveying tools and provisions. Gu Zhang did not come—he had already taken a position in the Ministry of War's Department of Military Affairs, specifically responsible for training young surveyors. He had recommended several apprentices to Shen Du, and these apprentices all accompanied him north this time.

Juyong Pass. Twenty years ago, when he stood here and drew the first map of the northern frontier defenses, several sections of the crenellations on the pass had collapsed, the guards' armor was worn and tattered, and only a dozen or so muskets were still firing. Now, the walls of Juyong Pass were entirely covered with blue bricks, the corner towers had been reinforced, the guards inside the pass trained in an orderly manner, and the muskets were polished to a gleaming shine. But he discovered that several drainage ditches on the west side of the pass were blocked by silt, preventing the rainwater from draining during the rainy season, and causing small patches of mold to appear at the base of the walls. "Dredge the silt. It must be dredged every year before and after the rainy season." Shen Du squatted down and dug away the silt at the entrance of the ditch, probing the depth of the ditch with his finger. "Have people been assigned to patrol the walls in the city? Guarding the city is not just about guarding the walls; there must also be people patrolling the base of the walls regularly." The commander of the pass knelt on one knee to receive the order, his expression a little tense—the old commander with gray hair in front of him still had the blueprints he drew twenty years ago hanging on the wall of the Ministry of War. Shen Du helped him up and led him on a tour of the city, pointing out the problems he had discovered one by one.

Datong. In the agricultural settlements of Datong, he saw that the wheat seedlings were growing well, and the water in the irrigation canals was drawn from the upper reaches of the Sanggan River, with the canals neatly paved with bluestone. However, on the east side of the settlements, the wheat in one field was sparse and the leaves were yellowing. Su Wanqing had already mentioned in her report that the local area needed to send more experienced water conservancy officials to be responsible for daily maintenance and improvement. Shen Du squatted down to look at the soil and tasted the water in the canal—the water was bitter. The local groundwater had a high salinity, and during spring irrigation, the water supply was insufficient, and the amount of irrigation water could not keep up with the evaporation. The salinity was pulled to the surface by the water and accumulated in the root zone, burning the wheat seedlings. "Is there any area nearby where we can replace the soil with sandy loam, or dig deep wells to replace it?" The local old farmers shook their heads and said no, but there was an old riverbed nearby that could store water during the rainy season. Shen Du led his men to the side of the old riverbed and dug a few feet down with shovels; the soil was wet. "There is groundwater beneath the old riverbed. If we dig a few shallow wells here, we can pump water to replenish irrigation during the dry season, and the saline-alkali land can still be saved." He drew the locations of the wells on a map, clearly marking the surrounding similar areas, and left it for the local garrison.

Xuanfu. Shen Du stood on the city wall of Xuanfu, looking north. The sky over the grassland was hazy. This spring, there had been little rain, and the grass on the grassland was turning green almost half a month later than usual. He noticed several new springs on the edge of the grassland outside the city gate—springs that hadn't been there last autumn. The spring water overflowed the ground, forming shallow puddles on the grass. "The distribution of water sources on the grassland has changed. Where there are many springs, the grass is good, and nomadic tribes will naturally gather near the water sources. But we don't have these springs on our old map north of Xuanfu." Shen Du pointed to the location of the new springs and said to the young surveyor beside him, "Mark these springs on the new map. Also, set up a few more hidden sentries near the springs to prevent Tatar cavalry from sneaking in and attacking. At the same time, keep in touch with the Doyan Three Guards—Huozhen's son, Batu, is now the commander of the Doyan Left Guard. His men are most familiar with the grassland's water sources and can assist with patrols."

The young surveyor quickly scribbled on the paper with charcoal, then looked up and asked, "Lord Li, the remnants of the Tatars no longer dare to move south, so why are we keeping such a close watch?" Shen Du put down his binoculars and looked into the depths of the grassland: "Because the enemy won't remain weak forever. You see, the grassland is quiet now, but quiet doesn't mean peace. Although the Tatar remnants have scattered, they are still here. Although the Oirats have submitted, they are still watching. Whoever can gain a foothold on the grassland will become the new threat. We build walls, fortifications, and hidden sentries not to block today, but for tomorrow." He turned around and wrote four characters in the blank space of the survey map—"Still waters run deep." The young surveyor wrote these four characters on the paper, and also in his heart.

Shen Du stayed in Xuanfu for three days. He retraced all the passes and military settlements from Juyong Pass to Datong and then to Xuanfu, recording all the problems he discovered in his memorial, and attaching the newly surveyed maps to the back. He stood on the highest city tower in Xuanfu, watching the sunset over the grassland—round and red, hanging in the sky like a giant lantern, dyeing the entire grassland a dark gold. He suddenly realized that this was the first time in decades that he was standing here not because he was rushing to the battlefield. In those years, every journey he took was connected to the next mission—forced marches, expeditions, beacon fires, and military orders to arrive within a specified time. He had no family, no children, and never stayed in one place for more than three months. He had made himself a nail, driven into every inch of the frontier. Now his hair was white, and his legs were lame. But that nail was still there.