The Walls were built more than a century ago, when humanity was brought to the edge of extinction at the hand of the Titans. In response to this looming threat, the humans made huge walls to protect their last stronghold. The walls reach about 50 metres in height and reach four or five times a human's height below the ground, and is rumored to be as strong as bedrock. However, the Female Titan was able to damage them when she attempted to climb over it, and the Beast Titan was able to remove pieces of Wall Rose and throw them. It was discovered by a miner that spent twenty years attempting to dig under Wall Rose to reach Wall Sina that after digging to the bottom, he found that the ground below them was made of the same material as the wall. They also seem to extend surrounding human territory underground.
The walls are too long to be guarded in their entirety. The tops of the walls are lined with cannons, which can fire on approaching Titans, even though they are not as effective due to the Titan's regeneration abilities. To account for this, each one has four towns on their periphery. As Titans are drawn towards large concentrations of people, humanity can focus its defenses at these towns and not the rest of the wall. These towns are where the poorer citizens of humanity live, since they are at greater risk of a Titan invasion.
Few people know about the Titans within the Walls, or that they were created by the Titans.  The Survey Corps learned of their existence when the Female Titan damaged the surface of Wall Sina, exposing the face of a huge Titan. Pastor Nick, a foremost member of a cult which considers the Walls to be holy, knew of the Titans within them, but would reveal no more under the interrogation of Hange Zoë. It is unknown how commonplace this knowledge is within the Wall Cult.
|Wall Maria is the outermost wall of the human kingdom. Its radius is 480 km. Like the other walls, Wall Maria is approximately 50 meters in height. The distance between Wall Maria and Wall Rose is around 100 km. Five years ago, it was breached by the Colossal Titan and Armored Titan, after about 100 years of peace and prosperity, and was lost to the Titans.|
|Wall Rose is the second outermost wall following Wall Maria. It is 380 km in radius. The distance between Wall Rose and Wall Maria is 100 km, and the distance between Wall Rose and Wall Sina is 130 km. It has been breached by the Colossal Titan after 5 years of the Wall Maria being breached, but has been covered by a boulder with the help of Eren Yeager. It was thought to have been breached again after Titans appeared inside it.|
|Wall Sina is the innermost wall protecting the kingdom where the king and other civilians reside. It is 250 km in radius. The distance between Wall Sina and Wall Rose is around 130 km. The city inside Wall Sina is protected by the Military Police serving under the king.|
- ↑ Shingeki no Kyojin Manga: Chapter 37 Page 8