Unholy Heights follows the adventures of the Devil who, in a last ditch effort to take over the world, has spent the last of his savings on a dilapidated tenement building.
When the game begins, the Devil has converted the building into monsters-only housing, and you must recruit visiting monsters to live in your building so you can extract rent from them.
This, naturally, attracts the ire of nearby humans, and so it's up to you to enlist your tenants in defending your building from warriors, pirates and villagers who want nothing more than to pillage what little gold the Devil has left.
Unholy Heights is split into two types of core gameplay: apartment management and apartment defense.
When monsters approach your apartment, you can simply click on them to move them into one of your vacant apartments. Once they've been moved in, you can click on the room they live in and purchase furniture to improve their satisfaction of both The Devil and the room they live in.
Each monster is different, and some desire certain types of furniture in their room before they'll be satisfied. While monsters will pay rent, they won't purchase their own furniture. That's up to you.
Clicking on the sign board next to the building will allow you to take on quests, which when completed will unlock new monsters, furniture, and harder quests. During quests, waves of heroes will invade the building, intent on murdering tenants and looting The Devil's penthouse.
You can knock on tenants' doors by clicking on them, calling them out to fight the interlopers. You can also click on them again to make them retreat back to their homes. Using these two simple commands together, you can juggle units around, avoiding focused damage on a single unit.
Depending on the time of day and the current weather, different residents will get stronger or weaker. Water elementals gain considerable strength when it rains, while cheepies gain strength in the sunlight.