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 from base to summit and are incredibly strong in substance. 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.

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 Sheena, exposing the face of a huge Titan. Minister 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 hands of the Titans "almost overnight". The anime states that in response to this, the survivors made huge Walls to protect their last stronghold. However the manga seems to indicate that the Walls existed before the Titans did.

The current status of the Walls is chaotic. Shiganshina District was the first city to fall when Wall Maria was breached, leading to a massive slaughter and prompting humanity to abandon the entire perimeter and the remaining cities along the wall as they attempted to consolidate their losses. After the fall of Wall Maria, humanity was pushed into Wall Rose. Soon after a massive campaign to retake Wall Maria was launched as nearly 250,000 conscripted soldiers were sent out to wipe out the Titans; this campaign ended in disaster and saw the near-total annihilation of the army tasked with this mission. Within five years after that, the Trost District was breached, though it ultimately survived due to the intervention of Eren Yeager, while also ensuring the local population's survival. Some time later, Wall Sheena and the Stohess District were heavily damaged when Eren did battle with the Female Titan. 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 Colossus Titan appeared and caused major damages to Orvud District.

After five years of abandonment, the Survey Corps returned to Shiganshina District and successfully sealed the hole in Wall Maria. However, the mission to retake the Walls is not over yet.

Anime and Manga Differences
In the anime, when Titans suddenly appear, the survivors of the attack make a walled stronghold for humanity to reside in. But the inside covers of the manga suggest that the walls existed some time before the Titans appearance. This has yet to be explained or elaborated on in the main storyline.

In the anime, a story elaborating on the nature of the walls is given in Episode 25. It was discovered by a miner with twenty years of experience who attempted to dig under Wall Rose to reach Wall Sheena and after digging to the bottom, that the ground below them was made of the same material as the Wall and that they also seem to extend surrounding human territory underground. In the manga, Roy tells a similar story to Hange Zoë, giving some validity to the anime-only event.

In an image given in the first episode of the anime, the distance from Wall Maria to Wall Rose is said to be about 100 km, the distance from Wall Rose to Wall Sheena is about 130 km, and the radius of Wall Sheena is about 250 km. Assuming the walls to be perfect circles, this would make the total area inside the walls 723,823 km². In the manga, no exact measurements for humanity's territory are given.

Attack on Titan: The Harsh Mistress of the City, Part 1
Quinta District is a located on the western side of Wall Maria. It somehow survived the initial Titan attack from year 845, but was abandoned six months afterward.

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.
 * In the early draft of chapter 1 presented by Isayama to the editors of the, Wall Maria and Wall Rose were switched, so Wall Rose was the outermost wall breached by the Colossus Titan, and Wall Maria was the second outermost wall to where the survivors retreated.

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