Ties

Disclaimer: I do not own any of the Attack On Titan characters, nor do I own you. All Attack On Titan characters belong to Hajime Isayama, and I only own added characters.

Year: 845

The last territory left to humanity after the Titans appeared is surrounded by three concentric walls. Starting with the outermost, they are: Wall Maria, Wall Rose, and Wall Sina. In the most protected location at the center of the three walls is the Royal Capital. It is the center of all human activity, including the government and the economy. Around the Royal Capital stand rows of splendid palaces. The people who live there are assured a luxurious lifestyle.

But...

Even in the Capital, amid all that splendor...

No, because of all that splendor, there exist dark, stagnant places. A city surrounds the Royal Capital... and beneath it lies a vast living space.
The Underground. According to old documents, at one time humanity considered living Underground to escape the Titan threat. In the end, this exodus was called off, and vagrants and criminals moved into the ruins that remained. The deepest sections became slums, abandoned even by the by the Royal government.

This is one of their stories....