The Wall Titans were Abnormal Titans similar in appearance to the Colossal Titan, one of the Nine Titans. They possessed the ability of hardening and existed in the order of the millions within the three Walls, before Eren Jaeger freed them in the year 854 via unhardening the Walls.
In the year 743, King Karl Fritz used the power of the Founding Titan to create and command tens of millions of these Titans to create the three Walls (Maria, Rose, and Sina) and protect the last Eldian territory on Paradis Island. The lack of sunlight inside the Walls has rendered them immobile ever since, although King Fritz threatened to release them and use them to flatten the Earth should Paradis Island come under attack.
Their existence was first discovered when the Female Titan tried to climb Wall Sina to escape from the raging Attack Titan and the Scout Regiment.
Appearance: The Wall Titans were similar in appearance to the Colossal Titan in that both their surfaces were covered primarily in muscle tissue and lacked skin (although the Colossal had exposed bone over the muscle tissue of its form). They had proportionally small eyes and lip-less mouths which exposed their teeth. Their exact heights are unknown -and likely varied somewhat- but they were about the height of the Walls, which were 50 meters tall.
Abilities: Like other Titans, the Wall Titans generated great amounts of heat, enough to emit large plumes of steam like the Colossal Titan. They could use this ability both offensively and defensively, creating smokescreens and flash-boiling seawater hot enough to scorch people to death and lift up countless ships meters above the sea surface. Each of the Wall Titans had the ability to harden not only the top layer of their skin, but much of their body, leaving them deprived of skin and hair, although it was probable that they only did so because they were commanded to by the Founding Titan.
History: In the year 743, after King Karl Fritz abandoned the conflicts of Eldia causing the Great Titan War, he fled overseas with most of the Fritz royal family and remnants of the Eldian populace to Paradis Island, Eldia's last remaining undisputed territory. He used the power of the Founding Titan to create and order tens of millions of Colossal Titans to form the three concentric Walls to protect the Eldians. The King sent a message to the people of Marley, demanding that the Eldians on Paradis Island must be left alone or else the Colossal Titans inside the Walls would "flatten the entire world".
Story: During the battle between Eren Jaeger and Annie Leonhart's Titans in Stohess District, Annie attempts to escape by climbing the Wall but is stopped by Mikasa Ackermann. However, it damages part of the Wall, and after the battle a huge chunk is torn off, revealing the face of a mysterious Titan. The missing chunk of the Wall is later noticed by some Scouts.
Source: Wall Titan (Anime), available under CC BY-SA 3.0.