Creation of the walls in Attack on Titan
Basically in the world of attack on titans, the ability to become titans is based on blood. Only ppl of Elderian can turn into titans, so it’s like a genetic thing. According to the story told, Ymir ( first titan ) made some deal with some Devil to obtain this power thousands and thousands of years back, anyone who’s ancestors goes to her is Elderian. Thus, there are the people who aren’t Elderian ( and can’t become titans. Therefore these Eldrian ruled over for a while and kinda abused the people that couldn’t turn into titans.
So, how were the walls formed?
How could fritz control the titans?
Where the mindless roaming titans come from?
Well, basically anyone from Eldrian blood line can be made into a titan through the use of syringe and injection thing. So, initially, there weren’t any mindless titans. After the king left, there was war, and the Marley are the ones who won. They then enslaved Elderianes and as a punishment, they’d make normal Eldrian humans into titans by the injection and throw them on the paradise island. Walls were already there when they started throwing these mindless titans And by the time of Eren, 100 years later you have millions of them roaming around.