Very good
OK, I stumbled on this a bit late, as the game is more than two years old by now. The new one (Gemcraft Labyrinth) beats this one any time. Still, it is a very nice defence game, and a great precedessor to what is in my eyes the best tower defence game on the net. I've played a few rounds and enjoyed it, but I guess I'm more for the new one. It just offers quite a lot more, both in graphics and in gameplay. In this Gemcraft 0, you get random gems. In gemcraft 1, you can choose them and that makes a difference. Still, the random effect can be nice (and surprising), and Gemcraft 0 has more story to it than its follow-up game. A matter of taste if you ask me.