Easygoing Territory Defense by the Optimistic Lord: Production Magic Turns a Nameless Village into the Strongest Fortified City (Light Novel) Vol. 1

The Doctor is In! Van, fourth son of a marquis, is just a toddler when he realizes he's been reincarnated. Thanks to his literal lifetime of knowledge, he's raised as a child prodigy--until his production magic manifests, and it's the last thing his snooty mage family wants to see! His disappointed father banishes him to a podunk town on the verge of collapse, yet Van can only see the place's potential. Can our hero's bastion of battlements build a better life than battle magic ever could?!