The people of Eldia have believed that titans have ravaged the land and have almost brought humanity to extinction hundred of years ago. To prevent extinction, humankind built three walls to keep themselves safe from the titans.
Wall Maria is the outermost wall in the Paradis Island. It was also the home of our main protagonists, Eren Jeager, Mikasa Ackerman and Armin Arlet. Wall Maria stands approximately fifty meters tall and has prevented Titans from entering the Shigansha district, which is the town located on the south of the wall.
That is, until the sudden appearance of a Colossal Titan in the year 845 that easily broke through the wall. This let the titans enter the human territory for the first time and has let the people to either die or migrate to Wall Rose.
Wall Rose is the second outermost wall after Wall Maria. It was breached by the Colossal Titan in the year 850, exactly five years after the initial fall of Wall Maria. But thankfully, it didn’t end up like Wall Maria as the hole was successfully closed with the help of Eren Yeager and the military forces.
There are four districts behind Wall Rose, the first is Utopia District located in the north of the wall. It is unknown what kind of district it is but it was briefly mentioned by Armin to distract Bertholdt Hoover for Erwin Smith to slice its torso and free Eren from their captive.
Wall Sina is the innermost Wall where the king and other civilians reside. The city of Wall Sina is protected by the Military Police that served the king while there is also the underground where the less fortunate and poorer part of the city where criminal activities are much more prone.
The district on the north of Wall Sina is called Orvud District, it was known for where Rod Reiss went after he injected himself with the Titan serum and turned into a humongous faceless titan. Stohness District is located on the east of Wall Sina, it’s known for where the Survey Corps figured out that the Female Titan is Annie Leonhart.
Capital Mitras, as the name is stated, is the capital of Eldia and located in the center of Wall Sina. Erwin Smith was brought into the Capital to seek an audience with the King and the Royal Government for the crime of alleged murders by the Scout Regiment.
His protests however, fell on deafs ears and people expected an execution and for the Scout Regiment to be disbanded. But it was part of a coup d’etat done by Erwin and Commander Pyxis to bring down the corrupt Royal Government and the false king.
The Underground is more notable in the Attack on Titan spin off or side story called “Attack on Titan: A Choice with No Regrets” that features the story of Levi Ackerman. Kenny Ackerman discovered that his sister, Kuchel Ackerman, has become one of the prostitutes in the brothel and one of her clients as impregnated her, resulting the birth of Levi Ackerman.