The Great Wall of China is the longest wall in the Real World, and, as its name cleary implies, is located in the country of China.
The Great Wall appeared in an episode of The Adventures of Super Mario Bros. 3, "7 Continents for 7 Koopas". When Bowser tried to conquer the entire Real World, he put Bully Koopa under control of Asia. Bully vandalized the Great Wall of China by spraying graffiti all over it. Mario later stole Bully's wand, and turned the wall into a giant dragon, which later attacked Bully. Ironically, this could be seen as damaging the wall even more than Bully did. However, it seen that later the wall and dragon both appeared at once, though it is not explained how.