The game: GemCraft: Chapter Zero
The premise: Tower defence game with a twist. Once you have your towers where you want them, you can place different gems to kill the various creeps. You can combine your gems to upgrade and have more powerful towers. Traps are also used.
The review: Really liked this game. Once you get into it, and understand the mechanics, it will easily hold your attention for a long time, which is good because the full game is HUGE. Mind-blowing-huge. Even when you've beaten the final level, there's still more gameplay if you wish. Even though this is a prequel to the already popular series of Gemcraft, this is still a great flash game worthy of a look, especially if you like the tower defence genre.