When you aren't using tower defense strategies to address the threat the human race provides, Unholy Heights works as a landlord simulator. Your tenants are monsters, sure, but they're monsters with needs and feelings. They'll need to eat, go to work, find somebody to love and be satisfied enough with their room to stay the night. If you don't cater to their individual desires, there might not be enough tenants to summon when the humans come a rabblin' to your property.
The game's Steam page notes over 20 different monster species and a quest system to unlock new furniture and creature types. It also suggests that monsters can be romanced with "inappropriately shaped erotic cakes," meaning that there's true potential for the best path to romance since Viva Pinata's interpretive love dances.