A great starting point for this game, three things I think it needs to make it even better is:
1) A bit of a challenge, I found myself just spamming both the mouse button and space to wipe everything out in seconds
2) I also think that you should have ALL upgrade levels available at the start, so players can focus on a specific type, eg. Focussing on armour to make your ship more of a tank, this way a player wont have to completely change their style if they usually specialise
3) Repeating levels, most shooters of this kind have a re playability that is added be replaying levels to get the gold star for completing it almost perfectly
and sorry about the length of the comment, but this is a great starting point 4/5
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