Element Line (2004)

Blurb:
Fourteen years after the disappearance of the great hero, Laolyth, the only barrier between the monstrous Rizoms and the destruction of humanity are a few brave soldiers and the Shields, huge walls surrounding the remaining cities. As the Shields tenuously keep the Rizoms away from the populace, only the powerful Guild guards traverse the walls as traders between the fortress-like cities. Kam, an orphan boy living in the city of Grisfynn, is preparing to become one of these soldiers, which would allow him to travel with merchant caravans across the world. But he harbors a dangerous secret that not even his beloved foster family fully realizes...Politics and high fantasy collide in this first volume of the action-packed Element Line.

Source: www.anisearch.com/manga/3180