The Castellan System

Leon Linden, Eldest Son of the Marquis, ostracized by family, is exiled to Greystone Keep in the Barrens, surrounded by Demons. He binds with the Castellan System, gains ability to construct weapons and repair defenses. He takes in Refugees, finds Aether Crystal Mine, and upgrades castle. Unwilling to be manipulated by family, he establishes himself in the Barrens, transforming the keep into a fortress, resisting Demonic Beasts and protecting people.