I actually rather like this game. Here's my review:
PROS
-Good Graphics: Very simple, yet not lacking in detail. They get the point across, and will work even on older, slower servers.
-Very Simple Game: You made a defense game. You didn't add in ultra violence or other needless accouterments, because you didn't have to. The bare minimum is fine for this game.
CONS
-A little too easy: Once you upgrade a few towers, and stack them up, you just have to buy a HP refill every now and then. I'm not a game designer, so you'll have to talk to someone else about content. this is just my honest opinion.
-Speed Control: While it was nice to speed things up, at 2x speed there isn't much change. If you could speed up a little faster, say 4x or 8x, that would be nice.
-Short: Only 8 levels and a tutorial. Maybe in a later update you can add some bonus missions or make the old levels have a harder difficulty option.
-Repetitive music: The music tends to get a little dull. If it had a different theme evry few levels, or some mixing up, that would be nice. Don't worry. I've noticed a lot of games do that.
Overall, a simple little game to pass the time. With a little time and care, it might shape up to be something really cool. Keep up the good work.