In the computer games Civilization and Civilization II, the Great Wall is a Wonder of the World that grants several powers to the Civilization that builds it.
In both games, the Great Wall automatically causes all of the Civilization's cities to have City Walls. In addition, all enemy civilization will offer peace during negotiations. In Civilization II, barbarians are a severe disadvantage while fighting your troops.
The wonder of the world takes 300 resource shields, and requires the advance of masonry to be built. It expires with gunpowder in the first game, and metallurgy in the second.
Most veteran Civ players place the Great Wall as one of the more important wonders, although if you are planning to defend by offense, it becomes less important.