If this had been the first ever TD game I have played you would have received 9/10!
It was a really good game, but it didn't bring anything new or exciting the the TD genre. After Bloons Tower Defense games came out and playing those for hours upon hours, this game just isn't that much different. Granted its in space and there are no monkeys lol but the gameplay is pretty much the same.
However I still give it a 6 rating and consider it a great game mainly do to your towers. The towers bring alot more strategy to the game then most TD games have. Dealing with percentages, poison, shock, rust....I thought the towers were much more intricate then in most TD games and that added a element to it that was very enjoyable.
I saw someone gave you beef for it being too "easy" but like you mentioned the first three levels allow you to build up skill points before the fourth level which increased greatly in difficulty. So I very much appreciated the first three levels being so easy.
Also I agree with Raicheous-Muffin in that your background was awesome I just wish it would have changed with each level...Adding to the graphics a little.
All in all it is a good game, it is just simply to similar to all other TD games, save the more intricate strategic towers.
Good Job, I'll be checking out your next game for sure!