Several hundred years ago, humans were nearly exterminated by titans. Titans are typically several stories tall, seem to have no intelligence, devour human beings and, worst of all, seem to do it for the pleasure rather than as a food source. A small percentage of humanity survived by walling themselves in a city protected by extremely high walls, even taller than the biggest of titans.
On that day, mankind received a grim reminder. We lived in fear of the titans, and were disgraced to live in these cages we called walls.
SHINGEKI NO KYOJIN