Endless Wall

The Endless Wall is a long wall with a pathway and is situated along the peaks of the Khiren Mountain range. It's encyclopedia entry reads as follows:
 * This massive, ancient edifice divides Ei-Jei and Morgal. Its origin and purpose are largely unknown.

History
The Endless Wall is located on the border between the country of Morgal and the Ei-Jei region, with the country of Sana forming much of the southern border. While obviously ancient, the true origin of the Endless Wall is largely unknown. It serves as part of the only accessible pathway to the Apollo Lens. The eastern end of the wall is marked by a tall tower, located northwest of the town of Tonfon, and serves as the only known way to access the top of the wall. While staircases in the tower originally allowed travelers to climb to the top, it has since crumbled, so Psynergy is required to climb the tower.

A short walk away from the tower's peak lies a large gate, which can only be unlocked by means of three Colored Orbs. From there, the pathway continues westward, eventually splitting into two paths. One leads to a dead end, while the other leads to Apollo Ascent.

Walkthrough
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).

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
The Endless Wall appears to draw inspiration from the Great Wall of China. Like the Endless Wall, the Great Wall runs in a general east-west direction, and serves as a general border between the Chinese regions and northern countries.

Trivia

 * The Endless Wall is the location of a well-known glitch that can allow the player to access areas that are normally cut-off during the later portion of the game.