Walls & Titans
Walls
The Walls are large structures that surround the territory where the remnants of humanity live, and protect them from the Titans that rule the world outside. They were built more than a century ago, when humanity was brought to the edge of extinction at the hand of the aforementioned Titans. The Walls reach about 50 meters in height and four or five times a human's height below the ground, and is rumored to be as strong as bedrock
Titans
They are a mysterious race of humanoid giants that appeared roughly 100 years ago (107 years is the most specific number given during Eren Yeager's training) and rapidly exterminated humanity to the point of near-extinction. If put into a calculable perspective, the Titans first appeared in year 743. Their origins, overall numbers, and their physiological workings remain a mystery to humanity at this point in the story.