How were the walls in Attack on Titan built?


If I remember correctly, in Attack on Titan, it's mentioned or implied a
number of times that humans built the walls to protect themselves from the
titans. (Feel free to correct me if I'm wrong on this.) If so then, how did
they manage to get the walls erected (which seem to be the result of somewhat
extensive work) without getting killed, if they were already under attack by
that point?

(Note: I'm only about halfway through the anime series, so I might just be
missing something that appears later.)




## Spoilers from the manga.

After the king's sanctum incident (ch50~65)

> (when Rod Reiss tried to make Historia eat Eren),

it is disclosed that

> The First King used his Coordinate power (control over titans) to force
> those titans to use their hardening power to shape up the walls, then used
> his Memory power to wipe the memories from the pre-wall era from humanity.

Why did he

> leave hibernating titans inside the walls

is unknown.

> he also created the Wallist Church to help him oversee the walls safety.




## Spoilers from the manga.

After the king's sanctum incident (ch50~65)

> (when Rod Reiss tried to make Historia eat Eren),

it is disclosed that

> The First King used his Coordinate power (control over titans) to force
> those titans to use their hardening power to shape up the walls, then used
> his Memory power to wipe the memories from the pre-wall era from humanity.

Why did he

> leave hibernating titans inside the walls

is unknown.

> he also created the Wallist Church to help him oversee the walls safety.

