I don't know why all these helicopters are so adamant in taking down my tower that they're willing to kamikaze themselves into it but fighting them off is pretty damn fun!
The aesthetics, sound and presentation are all really good and the gameplay is pretty fun too. As others have mentioned, the game could do with some autofire and I also think the game could use some rebalancing. At first it's extremely difficult to stop every helicopter but as your tower gets smaller, it gets too easy to camp a small chunk and survive a long time. Maybe you could make it so the tower gives you a score multiplier or a stronger weapon based on how much is left? Or maybe the game would be better without a tower at all? It feels like it could be removed without affecting the core gameplay loop much at all and I always ended up losing from getting hit rather than the tower being destroyed.
Anyway, those are just some suggestions. This is a really nice entry, good job! :)