The concept is linear, simple, and has already been done.
The guns are also basic, and the game results in you only ever using 2, the highest level machine gun you have to kill the small enemies, and the highest level rocket to kill everything else. I would quite call it 9 different guns, more like 3, with 3 levels of strength.
The enemies seemed well planned at first, sending them in, slowly combining them, and then it got repetetive. Worse than that, towards the end of the game it seemed like you had run out of ideas and tried to hopelessly spam foes against us in hopes of making us lose, but you made the player too powerful. The only item I needed was the shield, spam your upgrades on them after you have a level 2 minigun and you're set.
The game was too easy, or impossibly difficult if you got unlucky, and required very little skill or strategy. You get 2 stars for the effort, but I doubt I can be convinced to ever play a game like this again, it's simply not enjoyable at all the way you went about it.