As soon as it was light, Carl squatted under the east fence, hammering away at the weld points. The metal plates were wet with morning dew. He shook the water droplets off his gloves and shouted behind him, "Push the ore two meters closer, don't go down the middle path."

Chen Hao emerged from around the corner of the workshop, carrying two aluminum lunchboxes. "Changed your route?"

“There are footprints on the ground where the heat source appeared yesterday.” Karl didn’t look up. “Going that way is like walking into a sitting duck.”

Chen Hao stuffed a lunchbox into his arms, then picked up a broken iron rod and drew a line on the ground. "Let's bury sensor strips along this road, a feint to cover our tracks while secretly advancing."

“You’ve gone astray,” Carl said, taking a bite of his biscuit, his cheeks puffed out. “But I like it.”

Nana's voice came through the walkie-talkie: "East District Construction Team, the wind speed has increased, we suggest lowering the welding temperature by twelve degrees."

"Roger that." Carl tapped the walkie-talkie. "Your robot is more punctual than a foreman."

“I’m not a robot.” She paused for half a second. “I’m the system’s controller.”

Chen Hao burst out laughing, almost choking. "Fine, fine, you're so independent, you don't need to eat or sleep and you don't need to get paid."

As they were talking, Susan came from the direction of the storage room carrying a toolbox, a roll of tape in her pocket, and several signal cables draped over her shoulder. "The connector on the outer perimeter third stake is loose, I'll fix it on the way."

"By the way?" Chen Hao raised an eyebrow. "You didn't go to sleep until 2 a.m. last night, and now you're taking a long detour to work."

“I won’t take a detour.” She pointed to a small mound of earth at her feet. “We need to bury new alarm sensors here, and the wiring needs to be laid in advance.”

“You’re treating yourself like an electrician,” Carl said, shaking his head.

“It’s better than you guys thinking you’re blacksmiths.” She squatted down to tighten a screw. “Besides, I’m happy to do it.”

As they talked, the sun climbed above the watchtower. The sound of hammers echoed from the workshop as several veteran members took turns forging the ore. The new wall structure needed to replace the old wooden frame with high-density material, each piece requiring on-site cutting and welding. Progress couldn't be halted, nor could it be disrupted.

Before noon, the first section of double-layered barbed wire was erected. Karl and his men embedded ore fragments into the gaps in the frame, compacting them layer by layer like building a wall. This type of stone comes from the outer island; it is hard, heavy, and difficult to move, but it can withstand impacts.

"At this rate, we can take care of the east side in three days," Chen Hao said, wiping his sweat. "Then we'll move on to the south side."

“Assuming no one disturbs us.” Susan stood in front of the control panel adjusting the parameters. Several red dots appeared on the screen, indicating the locations of the newly connected sensors. “I’m now adding recognition logic to the system. Animals passing by won’t trigger an alarm, but a group of people walking side by side will trigger a level two response.”

"How do we divide it?" Chen Hao asked.

“Look at stride frequency and spacing,” she said. “Wild dogs run with a chaotic rhythm, while people march in formation in a regular pattern. Combined with a body temperature distribution model, the false alarm rate can be reduced to below five percent.”

"Sounds like fortune telling," Chen Hao grinned. "But I like things that are mysterious, the more mysterious, the more accurate."

Nana didn't refute, only saying, "The test data is out. At night, the positioning error for those approaching within 100 meters is no more than 3.7 meters."

"That's enough," Chen Hao nodded. "As long as we can gain thirty seconds of reaction time, it'll be fine."

The drill began at 3 p.m.

Chen Hao stood in the center of the command area and blew his whistle. "Red Shield activated! Everyone proceed according to plan!"

As soon as the words were spoken, the base immediately changed its rhythm. Susan strode to the medical corner, opened the emergency kit, and checked the medicines; Karl grabbed a wrench and headed straight for the back door of the workshop, which was a weak point in the defenses; Nana sat in the control room, her fingers rapidly sliding across the panel, switching monitoring views.

Chen Hao grabbed the walkie-talkie, made a quick round, and returned with a frown. "Susan took forty-seven seconds to get to her post, and the door lock on Carl's side got stuck."

“The hinges are rusty,” Carl muttered. “I’ve already oiled them.”

“Don’t wait until the drill to find out next time.” Chen Hao flipped through his notebook. “We don’t have that many ‘next times’.”

The second drill was much faster. Susan checked her equipment five minutes in advance and arrived in just twenty-eight seconds this time; Carl changed the lock and opened the door successfully on the first try; Nana's alarm system simulated an intrusion path and issued a warning thirteen seconds in advance.

“Okay,” Chen Hao said. “Let’s do it again.”

At the end of the third attempt, the time it took for everyone to be in position was 29 seconds.

“One second short,” Susan laughed, panting. “My shoelace came undone.”

“Nobody cares about your shoelaces on the battlefield.” Chen Hao put away his notebook. “But from now on, everyone must wear lace-up boots, and there will be two checks every day.”

No one complained. Everyone knew this wasn't acting.

As evening fell, the last section of the wall was welded. Karl took off his goggles, his forehead covered in a streak of grime and sweat. He stared at the weld for a long time, then tapped it three times. The sound was solid, without any hollow echo.

“Pass,” he said.

Chen Hao leaned against the wall, munching on dry rations. "We'll start setting up the second layer of the detection network tomorrow, and install the sound collectors as well."

“I’ve already programmed it.” Nana stood in front of the terminal. “People have voiceprint characteristics when they speak, walk, and breathe. The system will learn common patterns and automatically mark abnormal clusters.”

"What if they keep quiet?" Chen Hao asked.

“Then look at the heat,” she said. “One person dissipates less heat, but ten people standing together is like an oven.”

"Smart." Chen Hao swallowed the last bite of his biscuit. "Looks like we're really going to become an impregnable fortress."

Susan then walked in, holding several sheets of paper. "I've categorized and labeled the medicines and tools, so future resupply and distribution will follow the procedure based on the numbering."

"You've even set up a coding system?" Chen Hao took the form and glanced at it. "A3-b7, sounds like a recipe."

“It’s better than not remembering at all,” she said. “And it saves time.”

"What will we do with the time we save?" Chen Hao asked with a smile.

"Go to sleep." She glared at him. "Do you think everyone's like you, able to get up in the middle of the night to check the security cameras?"

"I was being responsible," Chen Hao said, puffing out his chest. "It wasn't insomnia."

"Is your way of handling things to write the schedule while yawning?" Carl interjected. "I've seen it three times already."

“That’s called dedication to work,” Chen Hao waved his hand. “Besides, if I don’t write it, who will? Nana can’t sign it for me.”

Nana calmly said, "I can generate a template."

“But you can’t sign ‘Chen Hao’s own handwriting’,” he sighed. “It’s hard to be a person, it’s even harder to be a leader.”

As night deepened, the patrol team changed shifts. The newly installed identification tags reflected the light; everyone had to swipe their cards to register upon entry and exit. All cameras were in recording mode, and the hard drive indicator lights in the corner flashed incessantly.

Chen Hao stood in front of the command console, watching the grid on the screen move steadily. Full-area monitoring was activated, risk areas were marked in red, and everything was normal.

"Do you think they'll come?" Susan had appeared next to him at some point.

"I don't know," he said, "but I'm not afraid if I come."

She nodded and turned to leave.

Chen Hao didn't move. He took out a piece of paper from his pocket and unfolded it. There were a few lines and circles drawn on it, like a sketch or a doodle. He looked at it for a while, then folded it up and put it back.

Lights were still on in the workshop in the distance. Karl was inspecting the welds, holding a testing instrument, walking step by step along the base of the wall.

Nana said softly, "The ground vibration sensor on the east side has been calibrated and the error has been reduced to zero."

"Okay," Chen Hao replied.

He picked up the walkie-talkie and pressed the call button: "Sector B, respond when you hear me."

"Received," came the voice on the other end. "Everything is normal."

He released the button and put the walkie-talkie back on the stand.

The wind whistled through the barbed wire. There was no moon in the sky; thick clouds obscured the starlight.

Chen Hao turned around to go rest, but stopped at the door.

He turned to look at the screen.

One of the red dots suddenly flashed.

It is neither a continuous movement nor a curve traversed by an animal.

It stopped there, motionless.