What do we all think? Are there titans in the walls? In all seriousness the walls likely serve either the purpose of keeping humans and demi-humans contained like a prison or cattle pen; or they are meant to keep monsters out and the labyrinths are the monsters tunneling in. I think there is a good possibility that there are already gods and their factions in other cities of reincarnates that are warring and this one was overlooked because of the god being considered a failure.
Side note, thankfully we didn't lose our Lvl. 44 Finger Technique, lock picking mama when they moved to the next district.