Walls

The Walls (壁) are large structures located on the island of Paradis that surround the territory where King Fritz fled with many of his people, the Eldians, and protect them from the Titans that roam the rest of the island. They were built more than a century ago.

Description
The Walls reach about 50 meters in height from bottom to top 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 mounted on rails which can fire on approaching Titans; however, they are not particularly effective due to the Titan's regeneration abilities. To account for this, each Wall has four towns on their periphery known as "districts". As Titans are drawn towards large concentrations of people, humanity can focus its defenses on these districts and repel the Titans more effectively. The districts are considered the worst 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 did not reveal any more information, even when interrogated by Hange Zoë. It is unknown how commonplace this knowledge is within the Church of the Walls.

History
One hundred and seven years ago, it is said that humans were brought to the brink of extinction at the hands of the Titans "almost overnight". In response to this, the survivors made huge Walls to protect their last stronghold.

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. 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.

Five years after that, 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 Stohess District were heavily damaged when Eren did battle with the Female Titan. On the same day, the population of a small village named Ragako was overtaken by Titans. After the military took the royal capital, 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. A prolonged battle with the Colossus Titan, Armored Titan, and the Beast Titan has ensured the Wall's protection, but at the cost of nearly the entire expedition and massive damage to the ruined city. The battle resulted in huge losses among the Titans residing within and outside Wall Maria, as well as the death of Bertolt Hoover at the hands of Armin Arlert, transferring possession of the Colossus Titan. With the battle over, the nine surviving members of the Survey Corps returned to Trost District.

After the military's victory over the warriors at Shiganshina, the next nine months were spent wiping out the surviving Titans within Wall Maria. This was done with minimal risk to soldiers through the use of the Titan guillotine, which was designed through the use of Eren's hardening ability. Their installation at Trost and other Walled cities finally paved the way for human settlers to repopulate their abandoned cities along the Wall. The Survey Corps then ventured beyond Wall Maria to find the outside lands largely unpopulated by Titans, and the Corps finally reached the ocean shore signalling that, for the time being, they had regained full control of Paradis.

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

 * 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 northern region as relatively cold, rarely exceeding 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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