Tongguan, founded in 196 AD, was built by Cao Cao's order.

But Tongguan is the barrier in the pass, Cao Cao's power at that time was in Yanzhou, Yuzhou, why build Tongguan thousands of miles apart?

What's more, not far to the east of Tongguan, there is also a Hangu Pass, why did Cao Cao abandon Hangu Pass and build Tongguan?

Because in this year, a major event happened, Cao Cao coerced the Son of Heaven to order the princes, and Liu Xie, who fled eastward, from Luoyang to Xuchang.

At that time, the warlords of Xiliang still had very powerful forces, such as Li Dao, Guo Aduo and others, who had always been eyeing Liu Xie and regarded him as something in their hands.

At that time, Liu Xie fled eastward, and the two also led troops to chase after them, but unfortunately they were intercepted by Cao Cao and brought to Xuchang.

Cao Cao's forces at that time were in Yanzhou and Yuzhou, and his enemies were Yuan Shu, Lu Bu, Liu Bei, Yuan Shao and other generations. If Li Dao and other Xiliang forces were to make him enemies for Liu Xie, and attack Yanzhou in the middle of the pass, then he would be enemies on all sides.

Therefore, in order to prevent the warlords of Xiliang, Cao Cao began to build Tongguan after welcoming Liu Xie.

Originally, in the east of Tongguan, there is also a pass, called Hangu Pass, Hangu Pass is also the gateway to the pass, in the pre-Qin period, Qin State relied on the danger of Hangu Pass, I don't know how many times to block the attack of the six countries.

Originally, Cao Cao could rely on Hangu Pass to prevent the Xiliang warlords.

However, at the end of the Eastern Han Dynasty, the water level of the Yellow River had dropped, and the forest vegetation at the top of the thick mulberry plain, where Hanguguan was located, had been destroyed.

The water level of the Yellow River in the north has dropped, and the forest vegetation in the south has been destroyed, and the Hangu Pass has a flaw. Then there is the Hangu Pass that has been in disrepair for a long time, so the Hangu Pass in the pre-Qin era has lost the role of natural insurance.

If the enemy attacks from the east, Hangu Pass will have some effect. However, if the enemy attacked from Guanzhong to the east, the defensive effect would not be ideal, so Tongguan, which was located at the western end of the Confuse Road, was used as a substitute.

Tongguan is located on the loess plateau called Linzhiyuan, and like the thick mulberry plain where Hanguguan is located, Linzhiyuan is also flat at the top, and extends from the Qinling Mountains, Huashan to the bank of the Yellow River.

However, unlike Hangu Pass, Hangu Pass is located in the thick mulberry plain, while Tongguan Pass is located on the Linzhi Plateau at an altitude of 550 meters.

On the east side of the Intoe Plateau is a deep gully called Yuanwang Ditch, and on the west side is a deep gully called Forbidden Gully.

There is water in the second ditch, and the walls become like knives under the repeated erosion of water flow and rainwater.

When Tongguan was first built, the enemy from the direction of Hangu Pass wanted to pass through the Lintoe Plateau, and then climbed to the top of the plateau, then went down the forbidden ditch, and then followed the Tongshui to the confluence of Tongshui, Weishui and the Yellow River before entering the Guanzhong. Therefore, the Hantong Pass was built on the road that must be passed through the lower plateau.

But, as a general rule, it is always unwise to defend an important city by relying first on the periphery of the passes, and it is always unwise to resist it on the basis of the city alone.

Therefore, if you want to board the loess plateau that Tongguan refuses to guard, you must pass through the first gateway of Tongguan - Jinqiu Pass. But even if you break the golden steep pass, there is a narrow road called Huangxiangban in front of you.

If you want to climb the Lintoe Plateau, you can only pass through the Yellow Lane Ridge.

This road is the same as Hangu Road, the south is close to the plateau, there is a high bank uplift between the north and the Yellow River, forming the situation of the plateau in the south and the absolute stream in the north, there is only one hole in the middle, only a few meters wide, only for bicycles, and it is also a pill mud plugged.

The locals also call it the "five-mile secret door", "five-mile" describes its length, and the "secret door" describes its steepness. Because the road is dangerous, it is very suitable for ambushes.

This is the fortification of the east of Tongguan, there are Jinding Pass and Huanggangban as a barrier in the east of Tongguan, and it is difficult to attack Tongguan from the east.

And attack Tongguan from the west, it is much simpler, west of Tongguan, there is no other city pass, narrow road, soldiers and horses come from the west, you can directly board the Lintoe Plateau, arrive at Tongguan City.

But Tongguan can be called the second pass in the world, and it is not so easy to attack, its city walls are tall and strong, and it is very majestic.

If you want to bypass Tongguan and go to the Central Plains, you have to pass through Huanggangban and Jinding Pass, so that the defenders of Tongguan can kill behind the enemy and block the enemy in Huanggangban and annihilate them.

Therefore, if you want to enter the Central Plains from Guanzhong, you must take Tongguan.

On the west side of Tongguan, the top of the forbidden ditch.

Sima Yi has arrived at Tongguan.

He stood on the top of the mountain in the forbidden ditch, surrounded by Cao Zhang and Hao Zhao, the general of Tongguan.

Hao Zhao was originally stationed in Chencang, and later Cao Zhen ordered the soldiers and horses to retreat, Hao Zhao returned to Luoyang, because Hao Zhao was good at defense, Cao Zhen sent Hao Zhao to Tongguan as the Tongguan guard.

Under the feet of Sima Yi and others, there is a forbidden ditch with a depth of more than ten meters and a width of dozens of zhang, which is as steep as a wall due to the perennial rain.

The forbidden ditch is a natural gully on the west side of Tongguan, which leads to Ba Shui, which is equivalent to the natural moat of Tongguan, but it is a pity that the sky is not beautiful, the length of the forbidden ditch can not surround all Tongguan, and can only protect the southwest and rear of Tongguan.

The enemy comes from the west, can be killed directly from the northwest to the west gate of Tongguan, if the forbidden ditch directly extends to connect the Yellow River, then the enemy has to climb the forbidden ditch to reach Tongguan, Tongguan east and west sides will be flawless, become a perfect city pass.

The forbidden ditch mountain is the highest place nearby, Sima Yi came out of the south gate, came to the top of the forbidden ditch, and observed the Han army camp.

Of course, from the head of Tongguan City, you can also watch the Han military camp, Sima Yi came here to observe the defense of Tongguan at the highest point.

The Han army camp was built not far from the east gate of Tongguan, with 20,000 soldiers and horses, and was commanded by Huang Xu.

Hao Zhao looked at the direction of the Han military camp and said to Sima Yi: "Dudu, do you think there are flaws in the Shu army camp?" ”

Sima Yi looked at the Han army camp and said in a deep voice: "This Huang Xu heard that he was born in Shengkou, and was later rescued by Liu Chan, and was accepted as a righteous son by Huang Zhong. ”

Sima Yi meant that he recognized Huang Xu's ability, saying that he had set up a good camp and had no flaws.

Cao Zhang, who was on the side, said coldly: "The Shu army is stationed outside the east gate, and now His Majesty has asked Zidan to withdraw his troops, so let's take him." We'll have to send someone to inform Zidan.

But Huang Xu led his troops to block outside the east gate, and the soldiers and horses couldn't get out of the city, so it was inconvenient for us to send someone to inform Zidan, and we couldn't pick him up. ”

Sima Yi groaned: "This is just a matter of defeating Huang Xu. ”

Hao Zhao smiled bitterly: "Didn't the governor just say that Huang Xu's camp was built well? He has 20,000 soldiers and horses, and my Tongguan garrison only has 20,000, so it is not easy to defeat him. ”

"With the Marquis of Yan Ling here, it's not easy to defeat him?" Sima Yi smiled and said: "Marquis Yanling's martial arts are rival to Zhao Yun, and tomorrow I will ask Marquis Yanling to go out of the city to challenge Huang Xu and give him a dismount." I have my own plan to defeat him. ”