Easygoing Territory Defense by the Optimistic Lord: Production Magic Turns a Nameless Village into the Strongest Fortified City Manga, Vol. 1

Van, the fourth son of a powerful noble, is only two years old when he remembers his past life as a salaryman in Japan. With his adult brain in a young body, he's considered a child prodigy...until he displays the "Production Magic" skill when he turns eight. In a family that favors offensive magic, this crafting skill is considered useless in battle, and Van is banished to oversee a tiny country town in disgrace. With only his personal maid, his father's butler, a knight, a rescued slave, ...