Walls play a massive part in Attack on Titan. Not only because they protect humans from the Titans but also because there are a lot of metaphors that can work with them.
1. Are The Walls a Cage?
For starters, it is the one that the main cast points out. How, instead of protecting them, the walls act more so as a cage. This is especially true when you find out more about the world due to the phenomenal world-building the author will soon do (and has done in the manga). This metaphor also works better because it emphasizes how many of them lack the power to fulfill their dreams as their lives are continuously haunted by these terrific beasts, otherwise known as Titans. They are, in the truest of senses, prisoners to fate. And the story is all about fighting that destiny.
2. The Composition of The Walls
The second is, of course, the most obvious one that the Walls are supposed to be used to protect everyone from the Titans. But as revealed at the end of Season 1, there is more to it than that. The fact that the walls are composed of Titans, similar to the hardening abilities that Eren possesses which allows him to harden things to block passages or to beat someone down harder. This adds to the mystery of Attack on Titan and makes it all the more alluring.
3. The Structure of The Walls
But stepping back from its meaning, let’s dive into the composition and structure of the wall. You should realize by now that the walls are divided into three sections: respectively, Wall Maria, Wall Rose, and Wall Sheena. But did you know that these contain a more hierarchical structure. The further in you get the Walls; you will realize that they are fit more for the nobility. And the poor live in the outermost walls where they risk the most danger. But in doing so, also have the most freedom out of the three.
4. The Titans First Attack
The Titans primarily target the South-most area of the Walls, making that the most dangerous place to live in, as compared to the Northern regions, which are considered the safest.