Okay, so I have Immortal Defense, and it's significant to go in to why this game is good--and make no mistake, it is, in fact, good.
Every single review you will read elsewhere will go on and on and on about the story and the writing. It's true, the writing is good, and the story is pretty interesting. But story isn't what drives a game, much less a goddamned tower-defense game.
What's best about this game is the overall progression. And to get into that, we have to look at how the game actually works.
Like any good (or even solid) tower-defense game, you have a preordained "road" which the opposing forces traverse, and you have space along this "road" to place semipermanent automated installations. Immortal Defense is little different in this regard--you can place your "towers" (which the game calls Points) anywhere on the map, so long as it's X distance from the Path your enemies travel, and Y distance from any other Points. Standard fare, nothing too different.
The big shit comes, appropriately enough, when you start the first map and have no ability whatsoever to place any Points. Which is okay, because you--as in your cursor, your interface with the game--have the capability to directly attack the enemy. When you are close to an enemy with your mouse cursor, you will automatically fire at it, regardless of which direction it is in comparison to the location of your mouse cursor.
That part's a little different, most tower-defense games don't let you directly intervene like that as a fundamental part of the game. But maybe you've seen that before, maybe you've played South Park Tower Defense Play or some of the other few that do that. Here's the interesting bit: in Immortal Defense, your cursor has a secondary attack that you can charge and release with the right mouse button. And the further you go in the game's missions, the higher you can charge this attack, and the more ridiculous the effect is. Shooting the enemy with your own hand becomes a very, very important part of the game, and the Points are there as support and to hold the fort when you're busy.
That's not to say the Points aren't interesting in their own right. There are only a few in the repertoire that do things that are standard tower-defense actions--one shoots where it sees an enemy, and one helps all nearby Points, but other than that, I can't think of one that has a basic method that's par for the genre. Danmaku points, once you get them, are profoundly notable in that they are super fun to play around with.
Ya ought to buy this game. As a guy who, without much hesitation, buys games on Xbox LIVE Arcade for around $10, I can say that this easily hangs with some of the best of 'em. At $2, it's a steal. If you're big into tower-defense games (Hi Neb), it would be well worth it at $10 (which is the price of the Crystal Defenders tower-defense game, which isn't nearly as well-made in any way). Unless you are dirt-poor and hardly have fifty cents to your name, I highly recommend getting this.
"You haven't told me what I'm looking for."
"Anything that might be of interest to Slitscan. Which is to say, anything that might be of interest to Slitscan's audience. Which is best visualized as a vicious, lazy, profoundly ignorant, perpetually hungry organism craving the warm god-flesh of the anointed. Personally I like to imagine something the size of a baby hippo, the color of a week-old boiled potato, that lives by itself, in the dark, in a double-wide on the outskirts of Topeka. It's covered with eyes and it sweats constantly. The sweat runs into those eyes and makes them sting. It has no mouth, Laney, no genitals, and can only express its mute extremes of murderous rage and infantile desire by changing the channels on a universal remote. Or by voting in presidential elections."
--Colin Laney and Kathy Torrance, William Gibson's Idoru