Summary: It's the age of mazes, named after demonic dungeon mazes appearing beside or under towns. All this mazes as dangeroud as they may be bring enormous prosperty too to the targeted cities near them. The main gets reborn in this other world as fief-keeper's-san of a lower noble (having a male demon-butler as servant). Due to his former Japanese life's experiences he can make unusual modern day advancements like fllor-heating or other stuff for war to improve their civilization and make his way to be a reknown fief-keeper...