Done well, needs more
Good idea, I've not seen a vertical scroller combined with tower defence elements. However it all seems to be very basic with little expansion on it.
Yes there is a 'Maingun'. This maingun is a crosshair on the screen that changes size slightly when you click. This changs the focus from shooting enemies to clicking on enemies, subtle difference, but noticeable.
Yes there are a varying amount of enemies. However they are all the same image with different colours. The varied flight paths are GOOD, I like this in the aliens. However the same image with different colours does not make them different enemies. This is easily solved though, if aliens with different flights paths also have different images, it bring the two elements together well.
Turrets are good, nice variety however your plasma cannon seems weird, having 1 damage and 10 fire rate yet being insanely slow. No explanations for what these stats mean, nor any indicator of upgrade status outside of numbers on the menu, this may seem weird given what I said previously, but why not change the colour of the weapon depending on it's upgrade status? Or give it an identifying, incremental feature like strips or battle scars or a larger muzzle or some addendas or something?
I think the only real problem with this game is that everything you put down in as features is really a standard, you have the wrong focus. Upgrades, multiple levels and a final boss are all standard with TD and most shooters, work more on your original idea and you shouldn't go far wrong.
I look forward to seeing more of this game from you.