Warning: contains spoilers for the Attack on Titan manga and anime!
The gigantic walls surrounding the remaining cities in Attack on Titan keep humanity safe. But when parts of the walls crumble revealing a horrifying secret, people have to grapple with a sense of danger coming from inside and outside the walls.
Hajime Isayama’s brilliant and terrifying manga Attack on Titan is a masterpiece in world-building storytelling that injects little tidbits of lore throughout the series and keeps readers rapt with each big reveal. Titans roam the world devouring stray humans and the only safety is behind gigantic walls. The main characters are ignorant of the world around them since the monarchy wiped the citizens’ memories using a Titan power and also kept a whopper of a secret.
It isn’t until Chapter 33 that the three concentric walls surrounding the cities are revealed to be constructed by thousands of Titans standing shoulder to shoulder. This won’t come as a surprise to anyone who has read the manga or seen the anime as it was revealed early on, but why do these Wall Titans stay put? These massive 45-meter (147-foot) Titans are encased in stone, but once a bit of the wall crumbles revealing the fleshless face of a Titan, the Survey Corps has to struggle with revealing to the civilians that they are constantly surrounded by massive Titans. If even one of the thousands of Wall Titans got free it could level a city.
But the Wall Titans stay right where they are. Titans are typically only active in sunlight. It’s theorized that Titans need sunlight for nourishment, since eating people (the Titan’s favorite way to terrorize humanity) doesn’t appear to provide sustenance. But Titans have been shown to move around and cause destruction in the dark too, just with reduced vigor. The Wall Titans seem to follow the same rules and don’t break out of their prison/protective barrier due to lack of sunlight.
When the first Wall Titan is revealed peeking one eye out from the walls, a member of a religion that worships the walls suggests covering its face so it doesn’t get any sunlight. This priest is later questioned but refuses to give up any information about the Wall Titans. Nearly a century prior to the main plot of the story, the King used his ability to compel Titans to do anything he wants, instructing them to link arms and turn themselves to stone to create the walls. The walls were intended to be both a prison and a defensive measure to the feral Titans outside the walls. The residents behind the walls (known as Eldians) were forced there by another race of humans, the Marleyans that despise their cousins with the ability to transform into Titans.
This becomes a larger plot point much later in the series when the protagonists set out to preemptively strike against the Marleyans. With the ability to call forth the Wall Titans, the Eldians could trample the world underfoot. The King threatened this a century ago but never acted on it, using it as a sort of nuclear deterrent without any intent to launch an attack. Eren is in control of the “Founding Titan” and has the ability to call forth this onslaught. Up until this point the walls were necessary to keep Titans out. Attack on Titan’s Wall Titans are equal forces of protection and destruction for people inside the walls, but they are also a powerful weapon to use on enemies. But the Wall Titans’ use, much like nuclear annihilation, makes the Eldians into the monsters many assume them to be and could wipe out all life.