don't tell me he's gonna enslave her father too? cause that would get awkward really fast... "good morning yukino, naked again I see. hi sophia in a nightgown, hello sophia's father/my old friend/father-in-law?"
@Tzion iirc the dungeons aren't permanent, but the former demon lords' dungeons stick around cause their owners were so strong, like an image burned on a tv screen...if the dungeon core was a tv instead of a giant home/storage space/laboratory construct. probably if they fought in one of those dungeons it would collapse and at best all the treasures and knowledge inside would be lost and they'd be spat out outside anyways, at worst, everyone inside at the time might be scattered across space and time or even outright deleted from existence.
it's less of a loss to rebuild part of a city than to lose the entirety of a demon lord's heritage and possibly everyone involved in the demon subjugation as well
or if they used a living person's dungeon instead, either it'd be too weak to contain the demons (will and power of the builder vs the demons) and/or there's no telling what they'd do with the soul bits used to build it in the first place, after all that's how sohpia ended up a slave.