Nice Game
Its a good game, but you can win very fast, in the first levels i already had the laser and then i upgrade the shield and nobody could even touch me. Also, the timer its horrible, try to make more enemies come because it gets to a point that you are like a LOT of time per wave and it gets very boring. You can make different type of enemies to make this, not only that they change their hp, maybe someones can hit from distance, etc. I like the variety of weapons. And the last thing would be, i saw that the yellow circle (cant remember name :P) gave you money and hp. Try to add more of those, speed buff, double damage, etc. I think those are the only things you could improve.
Apart from those things, great game, if you make a third part i know it will be perfect :)
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Snake