User blog comment:MeDeoPal/On the Origin of Titans and Shifters/@comment-114.125.41.118-20141204165214

In my opinion, the Reiss family is the key to the origins of titans. They did some experiment, probably to make a controllable group of titans acting as soldiers to invade other countries(there can't be only one country on a whole planet).

And the experiment failed, resulting in a mad titan that eats human, causing the eaten to become titans as well.

The three layers of walls were previously meant to be a fort to fend off any military advancement made by the other countries. It's built and kept underground. As the experiment failed, it is emerged using some mechanism and used to protect the citizens instead. That explains the sudden appearance of the walls.

The Reiss family whos the real royal family now lives inside Wall Sina. King Fritz is a mere puppet for them and by not being the royals, they can remain not being overthrown and their social status safe.