A few things to be improved
Gun - Give it either more powerups, or more expensive. Having max gun at such an early level is wierd.
Autofire belt - Make upgrading it more noticable. I honestly did not notice anything until it was level 40. Since it is a gun that doesn't hit very often, make it so it atleast 1hit kills everything. Since it doesn't its really not worth it at all.
Shield - Needs to be way more expensive. I had 980 shield by the time I was level 27 and the game was over.
Charge blaster - is fine, a nice little oh shit button.
And lastly instead of making things harder to kill, up the ammount of enemies that come at you in higher levels, just keep their speeds down.
If you feel like it, more levels. I love defend your... games that just go on and on and on.
Other than all that this was an amazing (albeit easy) game. A few improvements and it would be even better.