Walls

The Walls are large structures that surround the territory where the remnants of humanity live, and protect them from the Titans that rule the world outside. They were built more than a century ago, when humanity was brought to the edge of extinction at the hand of the aforementioned Titans.

Description
The Walls reach about 50 meters in height and 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 with twenty years of experience - who attempted to dig under Wall Rose to reach Wall Sina - that after digging to the bottom, the ground below them was made of the same material as the Wall. They also seem to extend surrounding human territory underground.

The distance from Wall Maria to Wall Rose is about 100 km, the distance from Wall Rose to Wall Sina is about 130 km, and the radius of Wall Sina is about 250 km. Assuming the walls to be perfect circles, this would make the total area inside the walls 723,823 km just over twice the size of real world Germany.

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 particularly effective due to the Titan's regeneration abilities. To account for this, each one has four towns on their periphery known as "districts". As Titans are drawn towards large concentrations of people, humanity can focus its defenses at these districts and not the rest of the Wall. The districts are considered the worst and least desirable places to live, as they are at greater risk of a Titan invasion.

Few people know about the Titans within the Walls, or that they were created from 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.

History
More than 100 years ago, humans were brought to the brink 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 current status of the Walls is chaotic. After the fall of Wall Maria, humanity was pushed into Wall Rose. Soon after that, the Trost District was breached. Then the population of a small village named Ragako was overtaken by Titans. Afterwards the military took the royal capital, then a Titan twice the size of the Colossal Titan appeared and caused major damages to Orvud District.

Attack on Titan: No Regrets
Every district within Wall Sina is said to have an underground town, which would make the total amount of districts within that wall 10. The underground is poorer and slightly more dangerous than Wall Maria. Since it's several meters below the surface, most people die because of the lack of sunlight, others because of dehydration, deadly infections or murderers.

Trivia

 * Titans primarily targeted the southern region, even before the loss of Wall Maria. As such, they are considered the most dangerous and least desirable places to live in.
 * The northern districts are considered the most desirable region, due to minimal Titan activity. However, the weather is brutally cold and makes life for residents very difficult. Hajime Isayama suggested they may have hot springs to make up for the harsh weather.
 * Isayama has described the region as relatively cold, rarely exceeding degrees even on the hottest summer days.
 * "Rose" was the East Germany's secret codename plan to build the Berlin Wall.

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