Wall Titan

The Wall Titans (壁の巨人) are unusual Titans that make up the Walls that protect humanity. They are first revealed when the Female Titan tried to climb Wall Sheena to escape from Eren Yeager and the Survey Corps. The lack of sunlight inside the Walls renders them immobile.

Appearance
The Wall Titans are similar in appearance to the Colossus Titan in that both their surfaces are covered primarily in muscle tissue and lack skin (although the Colossus has exposed bone over the muscle tissue of its form). They have proportionally small eyes and lip-less mouths which exposes their teeth. Their exact height is unknown, but seeing as they make up the Walls, it is implied that they are extremely tall.

Abilities
It is implied that Wall Titans have the ability of using their crystallizing powers to create massive Walls, as ordered to by the First King.

History
In year 743, after King Karl Fritz abandoned the Great Titan War against the Marleyans, he fled overseas with most of the Fritz royal family and remnants of the Eldian populace to the island of Paradis, Eldia's last remaining undisputed territory, and used the power of the Founding Titan to order tens of millions of Colossus Titans form the three concentric Walls Maria, Rose, and Sheena to protect humanity. The King sent a message to the people of Marley, demanding that the surviving Eldians must be left alone or the Wall Titans would "flatten the entire Earth".

Over 80 years later, Dina Fritz explained this story to the Eldia Restorationists.

The Female Titan arc
During the battle between Eren and Annie Leonhart in the Stohess District, Annie attempts to escape by climbing the Wall but is stopped by Mikasa Ackerman. She damages part of the Wall however, and a huge chunk is torn off, revealing the face of a mysterious Titan.

The missing chunk of the Wall is immediately noticed by Mikasa. The Titan within the Wall follows Mikasa's gaze, and Minister Nick calls for the immediate covering up of the Titan, as the sunlight eventually will enable its movement. After Nick ignores Hange Zoë's questions about it, he is nearly thrown off the Wall by her, but still refuses to reveal any details he might know because of the Church of the Walls' forbidden nature to disclose secrets.

The Uprising arc
In the Reiss Chapel, Rod Reiss explains how the First King used the power of the Founding Titan to form the three Walls which surround humanity's territory, though the Wall Titans were not mentioned.

Return to Shiganshina arc
After the surviving Survey Corps reach Grisha Yeager's basement, they learn from his books the story of the formation of the Walls as it was told to the Eldia Restorationists by Dina Fritz.

The Science of Attack on Titan
In Rikao Yanagita's analysis of the Attack on Titan manga, an estimation of the amount of Wall Titans residing within the Walls is given. Using the dimensions given in Attack on Titan: Before the Fall, Rikao estimates that, if the Wall Titans are all approximately 45 meters in height with a proportional 13-meter shoulder width with all Titans lined up shoulder to shoulder, there are approximately 246,000 Colossus Titans within Wall Maria and a total of 585,000 Titans within all three Walls combined. This conflicts with information given much later in the manga in Chapter 86, in which King Fritz's ultimatum given to Marley states that he would unleash the "tens of millions of Titans that sleep inside the Walls" if war was declared against Eldia. It is unknown whether the King's claim was accurate to the Wall Titans' numbers or was hyperbole.

Trivia

 * In the second compilation film, the Wall Titan's design is slightly altered to make it look more like its manga counterpart.

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