The Jiannan Army did not immediately launch a siege operation, and after setting up camp under the Yak City, almost all the soldiers of the battalion were resting, except for the 5,000 soldiers in the artillery battalion. Since the Battle of Nanzhao, the Shenwei Artillery Battalion has existed in name only, because the Shenwei Artillery was completely demolished after the war against Nanzhao. The gun commanders and gunners of the Genshin Might Artillery Battalion were also scattered into the battalions and became ordinary soldiers. Before this expedition, the artillery battalion was formed again, and the original gunners were able to return to their old jobs.

In the year since the withdrawal of troops from Nanzhao, Wang Yuan has never stopped improving the Shenwei cannon. In Chengdu, the entire duty of hundreds of craftsmen is to make and improve Shenwei cannons. The Shenwei cannon designed during the attack on Nanzhao was not small, but it had a fatal weakness that it did not have a long lifespan. Most of the Shenwei cannons can only be projected twenty or thirty times before they can only be scrapped, because the projection intensity is extremely high, and the key parts of the Shenwei cannons are all wooden, and it is difficult to withstand such a large strength. The shaft of the throwing rod and even the throwing basket were quickly worn out, and there were many times in the battle of Nanzhao that the machine shaft was broken, the throwing rod was broken, and the throwing basket was broken, resulting in the accident of killing and injuring more than 100 gunners.

After returning to Chengdu, Wang Yuan ordered the craftsmen to solve such a problem, and on the premise of ensuring that the range of the projection could not be affected, the problem of the life of the Shenwei cannon must be solved. The craftsmen suggested that metal components should be used for key parts to ensure that they were strong and durable, but this was expensive, and Wang Yuan did not have the financial resources to do so for a while. But not long after, Yang Guozhong sent a batch of tens of thousands of sets of tattered armor, swords and iron guns and other things, which solved Wang Yuan's urgent need. The reason why Wang Yuan has these rags under his command is that he values the metal that can be remelted in these tattered weapons and armor.

Although the number of iron pieces on these rusted weapons and armor is small, it is enough to cast more than 100 sets of axles and throwing rods for the improvement of Shenwei cannons. Although Wang Yuan didn't know whether the composition of these metals could ensure the long-term use of the Shenwei cannon, at least the metal components were much more durable than the wooden ones. The results of the experiment also show that after replacing the metal components, the Shenwei cannon can be fired 100 times, which greatly improves the service life.

In addition, the craftsmen have also improved the structure of the Kamui cannon. The original Shenwei cannon was too tall and had fixed components, which was extremely cumbersome. In order to counterweight the Kamui cannon, which is several feet high, not only is the ground extremely heavy, but it also has to be fixed to the ground with a wooden stake when firing, which is extremely clumsy.

As a result of the improvement, the Shenwei cannon adopts a detachable and assembled method, and after each part is built and formed, it can be transported by vehicle and installed on site. Because of the fixed wooden parts, two craftsmen and twenty gunners could quickly assemble the Shenwei cannon into a finished product in an hour. In this way, the mobility of the Shenwei cannon is ensured. It is precisely because of this that when he passed through the desert this time, Wang Yuan was able to carry fifty Shenwei cannons loaded in the cart to cross the desert. Otherwise this simply cannot be done.

More than 100 sets of metal components could have made 100 Shenwei cannons, but Wang Yuan only ordered 50 to be built. First, Wang Yuan thinks that there is no need for too many Shenwei cannons for the time being, because this thing consumes too much manpower and material resources, and each Shenwei cannon needs to be equipped with more than ten horses, two carts, and dozens of personnel from transportation to installation. Resources are limited, and it is impossible to put them all on the Shenwei cannon.

Another point is also based on the consideration of the Shenwei Cannon itself. There are only 100 sets of metal shafts and throwing rods, and the replacement of metal components is not permanent, under the high-intensity throwing, these unknown metal components will still have problems. Rather than spending a lot of manpower and material resources to carry 100 Shenwei cannons, if they are damaged in wartime, they will become waste, it is better to bring the other half of the metal components as a backup, so that the damage can be replaced and repaired, which is obviously much more cost-effective. Moreover, for the sake of the number of ammunition thrown, the number of thunderbolts is not enough to ensure the launch of hundreds of Shenwei cannons, and Wang Yuan will never use Shenwei cannons to throw stones, so wouldn't it be a waste of the service life of Shenwei cannons.

At sunset, under the slope of Yak City, the soldiers of the artillery battalion are leveling the ground under the hillside, digging an earth rampart platform, and preparing for the placement of the Shenwei cannon tomorrow. The Tubo defenders at the head of the yak city on the hillside watched the movements of the Jiannan army inexplicably, and they did not understand the intention of the Tang army to build a platform so far away from the city.

The defending generals immediately denied the possibility of placing siege equipment. At this distance, there was no defensive device of any kind that could threaten the safety of the city. The building throwing truck in the yak city is the trebuchet with the largest range in the world, which can reach more than 300 paces, but in the range of nearly 400 steps, it is beyond reach. The only possibility is that these high platforms were used by the Tang Army to stand on high platforms to observe the overall situation and command operations under the banner during the war. However, this method is too stupid, and the command of the command flag in wartime is to use a high pole to provoke the air, so that the whole army can know the military order in the melee.

The next morning, the secret of the high platform was finally revealed. On the long slope leading to the high platform, dozens of soldiers climbed the steps of the earthen platform with objects one by one, and then assembled them one by one. When the morning sun rises two poles high, dozens of earthen platforms are erected one after another wooden tower-like things. The Tibetans defending the city had already recognized that it was a trebuchet, and they wondered what was the use of setting up trebuchets in such a position more than 400 steps away from the city.

As the war drums rumbled, the Tang soldiers poured out of the camp and assembled hundreds of paces below the city, and a siege battle was about to begin. The 10,000 Tibetan defenders of Yak City were not frightened at all, and they prepared for this big battle in an orderly manner. The throwing tower on the inner side of the city wall was equipped with boulders, and the Tubo soldiers and horses at the head of the city were equipped with arrows and bows. Bows, crossbows, beds, crossbows, rolling wood, and stones are readily available, waiting for the attack of the Tang army.

Wang Yuan sat on the black horse with his waist straight, and the black big coat was draped behind him and hunted in the wind. The golden bright light armor glittered in the sunlight, and the red tassels on the helmet were bright and dazzling. Beside him, Song Jiangong, Liu Jun, Wei Guangzhong, and other generals all stood on horseback, and Gongsun Lan Zema stood a few feet behind Wang Yuan, looking at Wang Yuan's back in front of him, with a confused look in his eyes and not knowing what he was thinking.

"Dashuai, it's time to attack." Song Jiangong said in a deep voice.

Wang Yuan slowly pulled out the broken army sword given by the emperor at his waist, and the blade of the sword glowed with a cyan cold light in the sun.

"Liu Dehai, is the Shenwei cannon ready?"

"Hello Marshal, everything has been prepared, just wait for the Marshal's order." Liu Dehai shouted in a deep voice.

Wang Yuan showed a smile on his face, and shouted in a deep voice: "Very good, then it's up to you." As soon as the city wall was broken, the whole army charged, and fought a quick battle to occupy the city before noon. ”

Song Jiangong said in a deep voice: "Are you sure that the marshal doesn't need the ladder catapult archers to assist the soldiers and horses in attacking the city?" ”

Wang Yuan smiled and said, "There shouldn't be any need, I know your worries, but I'm afraid your worries are superfluous." Liu Dehai ordered the Shenwei artillery to start attacking the city. ”

Liu Dehai shouted a promise, rode his horse and galloped out, rushed to the bottom of the earth platform, raised the command flag in his hand and waved it violently, and shouted in his mouth: "Shenwei Cannon will start attacking the city immediately." ”

On the earthen platform, more than 20 gunners immediately went into action, and one gunner used an axe to split the wooden lid of the long box, revealing a dozen round black clay thunderbolts lined with straw. More than a dozen gunners shook the shaft and pulled down a section of the throwing rod that was raised high to fix it. The gunner carefully held a thunderbolt in his six-layer leather bomb pocket. A tall and round-shouldered gunner approached briskly with a sledgehammer, and slammed the sledgehammer into a semicircle with a wooden wedge. The iron hook that hooked the throwing rod was suddenly loosened, but when I heard a boom, the weight at the front end of the throwing rod fell violently, pulling up the back of the throwing rod violently. With a bang, the thunderbolt ejected out, quickly becoming a small black dot in the air, flying towards the head of Yak City over a distance of more than four hundred steps.

The defenders at the head of the Yak City saw the round ball falling from the sky, they didn't know what it was, and thought it was a round stone bullet specially made by the Tang Army. What is the use of this watermelon-sized stone bullet? Unless it hits someone, it is not lethal. More than a dozen Tibetan soldiers in the area at the head of the city where the thunderbolt fell didn't even pay attention to the stone bullet at all, and watched it fall from the sky and land above the city wall impartially.

"Haha, the Tang people really ......" A Tibetan soldier was only halfway through his teasing words, when he heard a loud bang, and the thunderbolt that landed on the city wall exploded violently like a landslide. For a moment, black smoke billowed and scorching waves of air churned. The exploding thunderbolt flew in a ball of iron.

The Tubo soldiers within Fang Yuanzhang Xu were overturned dozens of times, and the Tubo soldiers who were at the center of the explosion were thrown several feet away, and some fell directly under the city wall. More than a dozen Tibetan soldiers were pierced through their armor and flesh by the flying iron balls, and those who hit the vital point died instantly, and those who did not hit the vital point were also drenched in blood, and the wounded could not afford to fall to the ground.

"My God." The others who witnessed this were stunned in place, whether it was the Tubo soldiers at the head of the city or the Tang soldiers under the city, most of them had seen such a scene for the first time. The sound of the explosion made people's ears ring, and many people couldn't hear the sound in their ears for a while.

The horror had only just begun, and after the first thunderbolts, dozens of thunderbolts were already being dropped inside and outside the city. Although the targets of the projectiles were difficult to control, and some of the bolts landed at the head of the city, some inside the city, and some only outside the city walls, the explosion of all the bolts caused unpredictable consequences.

More than a dozen thunderbolts landed on the inside of the city wall, directly destroying a large area of the throwing tower behind the city wall. The sky fire fell, igniting dozens of houses in the city, and thick black smoke began to burn. In addition to blowing up the Tibetan soldiers on the head of the city, throwing them into the air, hitting the battlements, or directly throwing them down the city wall; It can also blow the city walls to the bare and destroy the fortifications at the head of the city. Unless the thunderbolts that fall outside the city are too far away, they can only blow up large craters in the ground and rain down mud and sand. Any thunderbolt that landed on the left side of the city wall or hit the outer wall of the city wall directly exploded a large hole in the wall.

A short round of bombardment turned the western side of the Yak City into a hell of blood and fire. Wang Yuan painstakingly developed and spent huge manpower and material resources to create the thunderbolt bomb, which was finally worth the price. The explosion on the yak city also announced the hot soldiers. The instrument finally stepped onto the stage of war, showing its terrifying power.