For the next 10 hours, Immortal Defense will still be available for free at Game Giveaway of the Day. There are a few restrictions, however, according to the developer, rinkuhero:
"Although it’s the full version of the game, there are some restrictions: they tell me (I haven’t actually confirmed this) that it can only be installed once.
Also, anyone who gets it free from that site will not have access to future patches of the game (which will include new levels, a medals system, level modding, new music, a graphics upgrade, and so on)."
Just another step in rinku’s ongoing experiment with indie game marketing, the results of which you can read about here on his forums.
Update: It’s over, y’all.