If you were a fan of the simple yet challenging video games that were ubiquitous in the '80s, Radiant is right up your alley. It's a 2D vertical-scroller that borrows a page or two from Space Invaders, Galaga, Asteroids, and others of that genre. You guide your ship through the galaxy, blasting away at a seemingly infinite number of hostile aliens (and space debris).
An amusing little plot adds to the engagement. The game also gives you a ton of weapons to choose from and upgrade as you collect more cash. The graphics are nothing special, but considering what the game is modeled after, no one should expect them to be. You can control your ship by D-pad/trackball, by number keys, or by touchscreen. All of the methods are responsive, though I usually choose touchscreen.
Gameplay is a lot of fun, and definitely addictive. You keep telling yourself, "Just one more level, then I'll stop," and the next thing you know, it's 3 a.m. and you forgot to eat dinner. You have three game modes to choose from: Casual, Normal, and Hardcore. With Casual, you can slowly work your way through, continuing if you get a "game over." With Normal, once you reach "game over," that's it (and the challenge is a lot tougher). Hardcore is the same as Normal, except you lose a life every time your ship is hit. If you beat the game one way, you'll probably want to start working on the next-hardest level.
The biggest flaw in this game--and it's pretty big--is that if you get a phone call in the middle of a game, it will restart and you will lose your progress. Can you imagine that happening on the last level in Hardcore mode? Any gamer would lose their mind. Also, being able to control when you fire would be nice; as it is, the ship always auto-fires, which I suppose adds to the challenge.
Radiant is fun, addictive, and challenging (in Normal or Hardcore mode). If you're into retro games, it's pretty much a must-have. Unfortunately, the phone-call bug is a genuine problem that hurts this game's rating.