Is there anything you would give up everything to defend? Immortal Defense is a story-driven strategy-based game in which you give up life to traverse pathspace and defend your home world from destruction.
The brief text sequences between levels carries the story--to fight in "pathspace," you have foregone your body and normal existence, and the concerns of the people who are sworn to defend seem increasingly irrelevant to you, in this very different existence. I typically think this kind of backstory is irrelevant (and often poorly written), but here it's almost haunting--nicely done.
A psychological, character-driven storyline. Each tower type is a different emotion or part of your mind. Level editor with Steam workshop to easily share and download new campaigns. Steam cards, hand-painted in watercolor, and achievements which grant gameplay bonuses.
Adjustable difficulty. Optional, hidden side campaigns. Secrets for the ages. Twenty six weird, unique types of enemies. The twenty hours of gameplay for you translate into millions of years of experience for Subject K.