The Harem is where you can change equipment, level up units, visit the Matriarchs and even interact with them in hot scenes.

You can visit each of the Matriarchs in order to get their blessings or watch sex scenes.

You can also visit any of your units, where you can level up them and their abilites. You can also watch sex scenes of those as well.

You can visit the Overlord's room to change the Overlord's equipment too.

The guest room is where Event characters will appear during specific events.