I really liked this game
For one thing, making a tower defense game entertaining... that's quite an accomplishment. It is an entirely different matter when that game is ingenuitive. I found the concept for the game to be addictive and fresh. The way you can combine the gems to create better and stronger versions... and thus make your towers better: excellent core gameplay!
It makes it even better by allowing me to constantly be doing something. I can control the speed of the game, destroy monsters, constantly be making gems, and then throw my excess gems to destroy monsters. Not once did I feel like I was bored or wasting time... which isn't good because I'll waste my life trying to beat this game.
I also like how you can beat the level, and still not have beaten it. The high score goals for each level are nice targets to aim for, and the secret levels to be unlocked are intriguing as well. I am also thankful for the save feature... I will come back to this game tomorrow!