The Garrison is one of the three main branches of the military in the world of "Attack on Titan." It is a defensive military organization primarily responsible for the protection and maintenance of the Walls that surround human territories.
The Garrison is tasked with the defense and security of the innermost Wall, Wall Sina, as well as the middle Wall, Wall Rose. Its primary objective is to ensure the safety and well-being of the civilian population residing within the Walls. The soldiers of the Garrison are responsible for patrolling the Walls, maintaining the infrastructure, and responding to threats within the territory.
Unlike the more adventurous and combat-focused Survey Corps, the Garrison's main role is defensive in nature. Its soldiers are trained in maintaining and repairing the Walls, monitoring for breaches or abnormalities, and maintaining order and security within the Walls' settlements.
While the Survey Corps ventures outside the Walls to confront Titans and explore the outside world, and the Military Police focus on maintaining order within the innermost Wall, the Garrison acts as the bulwark of defense against external threats.
The Garrison plays a significant role in the series, particularly during times of Titan incursions or crises within the Walls. Their efforts to protect the civilian population and maintain the integrity of the Walls are vital for the survival and security of humanity.