Endless Wall

The Endless Wall is a long wall with a pathway and is situated along the Khiren Mountain range.

Walkthough
Here is the start of the great wall that you've read about. Go up towards the door, which is locked, and grow the plant to the left. Now you can use it to climb inside (after freezing a puddle into place, of course).

Inside, you can move north to get around to the other side, where you can jump out the far right window to get that first chest you saw for some Lucky Pepper. You'll have to go back in though, of course.

Once you're back in, go down a single flight of stairs and jump over to the fire. Use Eoleo's Thermal Psynergy

With three orbs, we can now unlock the door at the top of the tower. Go past the door to get on top of the wall on the world map. Here you want to reach the Apollo Ascent, so follow the wall until it splits, Take the north path and look for the swirling darkness. Enter it to reach the Apollo Ascent.

Real World Inspiration
It is similar to the real world's Great Wall of China.