The Great Walls protected was finished in a matter of days. They stood erect and regular carriers and Hunters would have a hard time climbing the walls. The wooden gates were the only weak point of the walls and unless they were able to innovate how to make a movable solid iron gate, it would remain that way.
The garrison members have the task of defending the territory from all threats. They guard the Cemetery Dungeons and make sure the summoned carriers' numbers are low. so no dungeon breaks occur.
Aside from that, other members were permanently stationed at the Red Portals to guard against invasion attacks and welcome random strangers,
Additionally, they regularly visit the outskirts of the territory to make sure that no bandits would live off the territory.