The Great Wall is a unique tile improvement of the Chinese civilization in Civilization VI. It can only be built in a line on the current border of your territory and cannot be built on top of resources.
The Great Wall initially provides the same defensive bonus as a Fort, though it becomes available earlier and has more placement restrictions. It also provides Gold (and later, Culture and Tourism), so China's Builders should erect it along the perimeter of their territory as their empire's borders expand. They can maintain a strong defense against invaders by keeping it manned with units, and the bonus yields from adjacent segments can become quite impressive later in the game.
Civilopedia entry Edit
A fortified border took on new meaning when some civilizations attempted to make sure the uncivilized stayed out. A number of empires built "great walls" – the Roman Hadrian's Wall, the Chola-Pandya Madukkarai Wall, the Sassanid Great Wall of Gorgan, the French Maginot Line, and, of course, the Great Wall of China. Most were built of earth and stone, most incorporated natural terrain features (hills, rivers, and the like), and most had fortresses or garrisons scattered along their length. They were built in stages, sometimes over a long period of time. In terms of intent, none of these worked very well, for the outsiders managed time and again to "cross" the frontier walls, no matter how great. On the other hand, these great walls did provide a psychological sop to the empire's citizens, and did deter the less barbaric barbarians from raiding the frontier settlements. With history having passed them by, most of the great walls have fallen into disrepair and obscurity ... or become tourist traps.