Immortal Defense puts you in the role of a man whose soul has been separated from his body and cast into higher dimensional space. There, you shoot things on a line, then achieve enlightenment.
It breaks down like this: there's the timespace we all live in, then there's hyperspace, that side-ways highway where you can break relativity's speed limit and not get pulled over, and THEN there's pathspace. So its kinda tight; instead of just throwing you at some generic, tower defense scenario of defending against inexplicable monsters who just gotta follow that dirt road, you're given a perfectly believable justification for this RTS sub-genre, and its kinda a trip. Follow through with crazy procedural effects, towers that are actually representations of your personality (Fear slows, Pride grows in power for every kill, Courage shots keep going in a straight line) and a storyline that makes you question immortality, interventionist foreign policy, and the meaning of justice, and you've got yourself one tasty trip.
As a game, ID is balanced as hell, and keeps variation going through 100 levels. It's meaty, and it's all white-meat chicken breast at that--except it's seasoned, it doesn't taste like the Flash tower defense chicken you can get for free. It tastes like... enlightenment chicken. New enemies, new paths (some of which require a total change to the defensive patterns you're used to), new points (what this game calls "towers") and the relentless tug of the story get you going. It's addictive, and when you finish it, haunting.
Frankly, the making of Immortal Defense could mark the "topping-out" of the tower defense genre, taking everything compelling about its gameplay and commenting on it at a basic level. "Then again, I run a shooting gallery."
Ah, but thats not the son of a bitch.
Because it also comments on the relentless human drive to do something, anything, whether it be good or bad, just to feel like we've made a difference.
"I love you, Grandpa."
I think this was maybe the second game where a specific piece of text haunted me late, after completion. "What can change the nature of a man?" was the first - if you know what game that's from, you'll like Immortal Defense. If not... well shit man, whats your problem? It's Planescape: Torment. Now you know, go play ID, you'll like it.