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

Spring of the eighth year of the Yongle reign, Shanhaiguan.

Shen Du stood atop the Zhendong Tower, the highest point of the fortress, facing the grey-blue surface of the Bohai Bay. A sea breeze swept in from Liaodong Bay, causing the banners on the tower to flutter wildly. Behind him lay the tail end of the Yanshan Mountains, the Great Wall winding eastward from Juyong Pass, plunging into the sea here. This was the easternmost point of the Great Wall—Laolongtou (Old Dragon Head). The fortress beneath his feet was built during the Hongwu reign, its walls constructed of bluestone slabs quarried from the Yanshan Mountains, its foundation three zhang thick, and the five large characters "The First Pass Under Heaven" inscribed above the gate. However, the fortress built during the Hongwu reign had a flaw—the walls only reached the foot of the mountain; the passes above the mountainside were not completely sealed off. Northern Yuan cavalry could bypass the fortress via mountain paths and directly penetrate the Liaoxi Corridor.

Shen Du traveled from Beijing all the way to Shanhaiguan, surveying over 700 li (approximately 350 kilometers) along the way. He re-surveyed every pass, every beacon tower, and every mountain path leading to the border from Beijing to Shanhaiguan. Because the existing beacon towers were too far apart, visual signals were easily interrupted on rainy days. He added dozens of new beacon towers along the way to ensure that beacon fire messages could be transmitted from Shanhaiguan to Beijing within half a day.

"The troop deployment of the Liaodong Regional Military Command needs adjustment." Shen Du stood on the city wall, tucked a charcoal pencil behind his ear, and spread out a map to the Liaodong Regional Military Commander beside him. "Originally, we placed the main force in Liaoyang, but Liaoyang is too far from Shanhaiguan. If there is an alarm at Shanhaiguan, it will take at least ten days for the Liaoyang cavalry to arrive. It would be better to divide the main force in two—one part will remain stationed in Liaoyang, and the other part will be moved to Guangning. Guangning is closer to Shanhaiguan; cavalry can arrive in five days, and infantry in eight days. In this way, Shanhaiguan will have depth support, and the defense line will not be a thin line, but a flexible defense system."

The Liaodong Regional Commander stared at the heavily annotated map in silence for a moment. He was an old border general who had fought for decades, his hair completely white, his face etched with the deep lines of the harsh winds and sands of the frontier. He looked up at Shen Du, a hint of scrutiny in his eyes, but more so, surprise—this deputy commander from the interior hadn't come just to make a formality. "Lord Li, winters in Liaodong can freeze your ears off, spring brings sandstorms, and summer the mosquitoes are bigger than flies. Are you willing to stay and finish drawing this plan?"

Shen Du took the charcoal pencil off his ear and began sketching a plan for the expansion of Guangning City on the paper. "This is what I came here to draw."

Zhao Laoliu came to Liaodong with Shen Du. He hadn't even had time to settle into his official post at the Ministry of Works when he volunteered to accompany Shen Du on his journey beyond the Great Wall. His reason was, "Lord Li's legs aren't good; he needs someone to look after him on this long trip." But Shen Du knew his true intentions—he had spent six years building city walls in Beijing and now wanted to see if his work could be used on the frontier. Zhao Laoliu oversaw the expansion of the watchtowers at Shanhaiguan. He divided the craftsmen he brought from Beijing into three shifts, working day and night to lay bricks. They used bluestone slabs from the mountains for the base, bluestone for the walls, and lime mixed with glutinous rice paste for grouting. A watchtower and beacon tower were built on top. Halfway through construction, he discovered that the lime transported from within the Great Wall was insufficient. So, he led men to reactivate abandoned shell kilns by the sea to burn shell ash—more viscous than lime, and non-dissolving in water. Using shell ash to build the walls of the watchtowers by the sea was more durable than using lime from the interior.

"I heard about this seashell ash from the old fishermen in Liaodong," Zhao Laoliu said, his pipe in his mouth as he smeared a handful of freshly fired seashell ash mud onto the base of the pier. "They said it's what's used for ship repairs by the sea, and it's more resistant to salt water than lime. If we use this to build the pier, it will last at least twenty years."

Shen Du squatted down and looked it over. He stood up and said to the clerk behind him, "Write down the formula and firing process for the shell ash. All coastal garrisons will use this when building watchtowers in the future." The clerk opened the book and quickly wrote it down.

They stayed in Shanhaiguan for three months. During those three months, Shen Du led his surveying team to traverse every ravine in the Liaoxi Corridor. Twice they were targeted by small groups of Tatar cavalry, but Huo Zhen's young riders, brought from the Doyan Three Guards, were in their first combat experience and used bows and arrows to drive them back. Zhao Laoliu's watchtowers were built one after another, reaching halfway up the mountain. He traveled between the various construction sites every day, wearing out several pairs of cloth shoes, his heels cracked and bleeding from the cold. He would soak them in hot water at night and continue walking the next day. Su Wanqing came with a grain transport team and obtained a batch of sorghum seeds and salt-tolerant millet seeds from the Ministry of Revenue, which were distributed to the military households in the Liaoxi Corridor. She knew that walls alone were not enough; people needed to cultivate the land behind the walls, and only when people were well-fed could the walls be defended. Standing on the barren land covered in alkaline frost in the Liaoxi Corridor, she said to the families of soldiers who had come to collect seeds, "Sorghum and millet are more resistant to salt than wheat. The first crop may not grow well, but after two seasons, the land will be ready. You can try planting them first, and I will come to harvest the grain in the fall. The imperial court will buy as much as you have."

After the Liaodong Military Command was established in Guangning New City, Shen Du led a surveying team back into the pass and continued south along the coastline. From Liaodong to Shandong, from Shandong to Jiangsu and Zhejiang, from Jiangsu and Zhejiang to Fujian, from Fujian to Guangdong—this journey took another two years. The coastal garrisons and beacon towers were only in their infancy during the Hongwu reign, and large-scale expansion began during the Yongle reign. However, in the early Yongle period, he was busy with the Northern Expedition and the relocation of the capital, and there was no unified plan for coastal defense. Shen Du's mission was to map out this sea defense line from beginning to end: where to set up garrisons, where to build cities, where to store grain, where to mount cannons—he had to personally inspect, measure, and record every single place.

In Dengzhou, Shandong, he sifted through old files in the Dengzhou garrison headquarters, identifying twelve locations where Japanese pirates had landed during the reign of Emperor Hongwu. He then led Zhao Laoliu to survey each of these landing sites. Zhao Laoliu built a new type of beacon tower on the Dengzhou coast, topped with three copper pots—one for burning wolf dung in sunny weather (producing black smoke), one for burning dry firewood in rainy weather (producing white smoke), and one for lighting a lantern at night. These three signals combined could transmit simple intelligence. Shen Du named it "Three Smoke Tower," and it was later adopted by the Ministry of War for use in all coastal garrisons.

In Zhoushan, Zhejiang, he discovered that the garrison of the Zhoushan Islands was scattered across dozens of small islands, unable to communicate with each other. When Japanese pirates came, one island would be attacked without the others even knowing. He reorganized the garrisons of the navy and infantry, and established regular patrol boats between the main islands. In case of alarm, smoke signals would be lit on the main peak, and neighboring islands would immediately assemble upon seeing the signals.

Quanzhou, Fujian. Standing on the ancient wharf of Citong Port, watching the merchant ships of various countries shuttling back and forth on the sea, he said to the accompanying officials of the Maritime Trade Office, "The prosperity here depends on coastal defense. Without a strong fortress, the prosperity of trade is just a sand table that can be burned down at any time." The officials of the Maritime Trade Office agreed wholeheartedly and immediately expressed their willingness to allocate special funds from customs duties to help build a coastal defense post.

From Liaodong to Guangdong, they wore out countless pairs of boots, succumbed to heatstroke under the scorching sun, and survived capsizing storms. Among the young surveyors and craftsmen accompanying them, some fell ill, some fell behind, and some perished in the malaria-ridden coastal areas. At each new garrison, Zhao Laoliu would first discuss materials with the local craftsmen—blue bricks, shell ash, tung oil, bamboo fibers, hemp rope mixed with soil—their list of building materials grew longer with each change of latitude. The riders sent by Huo Zhen gradually became accustomed to living and eating with the inland craftsmen, occasionally discussing reinforcement plans for different sections of the city wall with Zhao Laoliu in broken Mandarin around the campfire. Su Wanqing's letters were regularly delivered from the Ministry of Revenue in Beijing to the post stations along the route, informing her of the progress of the coastal military households' land reclamation, data on the garrison's grain reserves, and the latest allocation of farm tools and medicinal herbs. She would occasionally slip in a question at the end of her letters: "Does your leg still hurt?"—her tone was as calm as still water, but she never missed a single one in any of her letters.

Shen Du would read each letter he received at night by the light of an oil lamp in the post station. After reading it, he wouldn't reply immediately; instead, he would fold the letter and tuck it into his pocket, writing a reply with charcoal the next day on the road. His replies were mostly about important matters—the beacon towers in Dengzhou were completed, the naval patrol system in Zhoushan was established, and the garrisons in Quanzhou were to cooperate with the Maritime Trade Office in defense. He only added one sentence at the very end: "I've run out of ointment; please send some more."

In the tenth year of the Yongle reign, when Shen Du returned to Beijing, he brought back not only maps of coastal beacon towers and garrison posts, but also a complete set of coastal defense strategies. At a post station outside Beijing, he transcribed and bound all the drafts from the past two years into a book, each page accompanied by precise measurement data and concise defense suggestions. This set of coastal defense strategies was later printed by the Ministry of War, serving as the foundational document for coastal defense during the Yongle reign. However, on the last page of the book, he wrote a sentence that Zhu Di did not approve at the time: "Coastal defense is not only along the coast. Coastal defense is at sea."

He knelt in the Fengtian Hall and explained this to Zhu Di: "The Japanese pirates come from the sea, and it's easier to intercept them at sea than on land. Fishing boats always spot them earlier than the pirate ships. Fishermen know the wind direction and currents earlier than the beacon towers along the coast. Relying solely on land defense is passive and vulnerable to attack. We must establish a patrol fleet at sea to actively search the surface, and recruit volunteer sentries and pilots from among the fishermen to assist the navy in early warning." Zhu Di listened in silence for a while, then picked up his brush and wrote two words—"Approved." This stroke marked the starting point of Zheng He's treasure fleet's initial coastal defense plan. At this moment, however, many years were still to come before the fleet set sail from Liujiagang.

In the spring of the eleventh year of the Yongle reign, in the Western Regions.

Shen Du stood on a stretch of Gobi Desert outside Jiayuguan, the snow-capped peaks of the Qilian Mountains gleaming with a golden light in the setting sun before him. Behind him were Zhao Laoliu, Gu Zhang, Huo Zhen, and a dozen or so young surveyors who had come from Beijing. Su Wanqing was not there; she had stayed in Beijing to oversee the spring planting for the new batch of military-household immigrants, only saying "Come back soon" before leaving. Before him, between the deserts and snow-capped mountains of the Western Regions, vast tracts of land yet to be mapped awaited his drawing.

Zhao Laoliu squatted on the Gobi Desert, picked up a piece of dried camel dung from the ground, weighed it in his hand, and turned to Shen Du, saying, "Master Li, I've counted along the way—from Liaodong to Guangdong, and from Guangdong to the Western Regions, we've worn out almost twenty pairs of boots."

Shen Du rolled up the blueprints and tucked them into his robes before mounting his horse. The horse was a newly acquired Western Region breed, taller and thinner than a Mongolian horse, its mane billowing in the wind. He glanced back at the team behind him—these young men who had followed him from Beijing, now possessed a greater sense of composure and calmness compared to the surveying team that had set out from Beiping years ago. The surveying tools had evolved from simple rulers and measuring tapes to more sophisticated instruments, but their work still involved traversing mountains and rivers, meticulously recording the terrain they encountered on paper.

"Let's go." He gently spurred the horse's belly. The caravan headed west along the foot of the Qilian Mountains, their hooves crunching on the gravel of the Gobi Desert, casting long shadows in the setting sun. Zhao Laoliu followed behind, pipe in mouth, humming a Liaodong folk tune he'd picked up from who-knows-where. From the Baigou River to Nanjing, from Beijing to the coast, from the coast to the Western Regions—the roads he'd traveled were longer than any line on a map. But he knew the journey wasn't over yet.