Published : 31/12/2025
The Great Wall, one of the greatest engineering marvels of ancient China, is a wall stretching for thousands of miles that holds countless civilisational codes, waiting to be discovered and deciphered.
A research team from Tianjin University has spent four years uncovering the Great Wall's little-known "secret doors". They identified over 130 ruins of these hidden gates across 10 provinces and regions, bringing these "secret passages", which have been concealed for centuries, out of the history books and into the public eye.
What were the secret doors of the Great Wall used for?
Secret doors, as the name suggests, are passages hidden from sight, mostly built in concealed sections of the Great Wall. The doorways were short and small, with the narrowest allowing only one person to pass through.
They were mainly used for the passage of scouts or for surprise attacks, while some were also used for trade.
Among the scouts stationed at the Great Wall during the Ming Dynasty (1368-1644 AD), the most legendary were known as the "Yebushou" (夜不收), which literally means "those who do not return at night".
Equivalent to modern special forces, they were primarily responsible for gathering intelligence on the enemy and conducting surprise raids. They earned their name because their missions required them to operate outside the wall throughout the night.
The passages they used to enter and exit the Great Wall were the hidden secret doors, and they had to verify a secret code to pass through.
So, how many of these secret passages are there in total along the Great Wall?
A research team from Tianjin University, over a period of more than four years, has confirmed the locations of at least 220 secret doors by consulting ancient atlases.
Spanning 10 provinces and regions, they utilised drones and digital technology to capture images of over 90% of the manually constructed sections of the Ming Great Wall, amassing more than two million photographs. This effort has enabled the creation of a "Panoramic 3D Imaging Platform for the Entire Great Wall," achieving image precision at the centimetre level.
From this, the research team from Tianjin University has found the physical remains of more than 130 secret doors and, for the first time, created a "family tree" of the secret doors.
These secret doors are typically 1.5 to 2.5 metres high, with the narrowest allowing only a single person to pass through and the widest allowing horses to pass. The construction of each hidden gateway varies, meticulously tailored to the terrain and designed to fulfil distinct functions.
"Sally Ports", the most mysterious secret doors of the Great Wall
The most concealed type of secret door is called a "sally port" (突門), which was mainly used for surprise attacks.
The side facing the enemy was camouflaged with brickwork, while the side facing our own forces was a hollow doorway where soldiers could be stationed.
In times of battle, the enemy could not discover the location of the sally port from the outside, whereas soldiers could break it open from the inside and launch a surprise attack.
The famous Qing Dynasty scholar Wei Yuan (魏源) once used the phrase, "Hidden beneath the Nine Earths is secrecy (暗); moving from above the Nine Heavens is a sally (突)," ("藏於九地之下為暗，動於九天之上為突") to describe the concealment of the secret doors and the cunning nature of the sally ports.
As early as over 2,000 years ago in the texts of MoZi (《墨子》), there were descriptions of facilities similar to sally ports, but no corresponding physical evidence had ever been found until 2019, when the research team discovered the remains of a sally port for the first time in Qinhuangdao City, Hebei Province.
Apart from their military function, some secret gates were also used for foreign trade, or as passages for soldiers and civilians on the border to go out of the pass to farm and graze.
Zhang Yukun (張玉坤), a research expert on the Great Wall's defence system and a professor at the School of Architecture at Tianjin University, believes that this "reflects the openness of the Great Wall" from another perspective.
"The secret gates hold the secrets of the Great Wall, and even more so, the wisdom of the ancients," pointed out Li Zhe (李哲), a member of the research team and a distinguished researcher at the School of Architecture at Tianjin University.
In the future, digital technology and other methods will be used to further restore the "true appearance" of the Great Wall and to continue to decipher the secrets of the secret gates.
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