the princess was used as a spark to start a war, lots of lives were lost, the people were angry at the princess and she was caught and almost executed.
They arrived at the dungeon where MC saves her and it was proclaimed that she died there to avoid anymore political strife.(nobles are using it to dethrone the king, other kingdoms are using her to destroy their kingdom and divide the loot/land)
Dungeon was discovered so the kingdom sent an army but in truth they just want to determine the princess is dead or kill her themselves.
Army was utterly crushed so to avoid another stronger force attacking the dungeon MC worked with the sister of the princess to proclaim she subjugated it and now controls it.
With that out of the way MC now wants to expand the dungeon and create an underground village to take in other survivors of the almost war.
Hence the slaves are called in as a sample and as a representative so he won't appear and make people know less about the true master of the dungeon.
as for why he is building a village it is super spoiler-ish and I am still waiting for an update of the arc(I know why but I won't tell after the arc ends)