Great entry! I enjoyed this a lot, especially the idea of the self-replicating enemy. It adds urgency to the gameplay, and thus fun. In my opinion the graphics and the sound, and maybe also the theme setting, suggest a different genre than "tower-defense": as other suggested in the comments, it's more an "arcade" game! I'm not saying that this is a problem, I was just surprised when I started playing :)
With the hope of being constructive, I tell you that having something that grows exponentially is often a risk in games. Exponential growth induces positive feedbacks: the larger the enemy, the faster it becomes even larger, and so on and so forth. That might be a problem for game balance, because a little error at the beginning - maybe you read the tutorial too slowly and the enemy grows too much - can soon ruin the whole game. Based on a true story :P
However, when I managed to destroy the first enemies, I had enough money to build more ships and the game became immediately easier, because I was shooting at them even before they start to replicate. In conclusion, it felt to me that the game was sometimes too hard, sometimes too easy. That's of course just my experience, I don't mean to tell you what to do with this :)
As a proof that I still enjoyed the game, let me flex my final score :P
I don't even know if it's bad or good, I was just proud to have lasted that much ahaha
Thanks again, and congratulations!