Agree with some, it's a nice concept and looks nice, but the combination of controls makes it unnecessary hard to play, together with the really slow progression money-wise.
It's also hard to keep an eye on the screen and carefully pushing the upgrade buttons at the same time. My suggestions:
- Implement autofire, that's pretty much what you do anyway
- Pause the game when buying upgrades, or alternatively be able to upgrade with a keypress (1 through 6) so you don't have to look for the right button to press
- Make it worthwhile to actually kill enemies (money?) instead of only do it to clear the screen
- Random upgrade or health drops
- Prevent enemies from appearing right on top of the player
Quality of life / bug:
- You can easily get stuck between enemies and the barrier, perhaps make you unhittable for a few seconds whenever you get hit so you can escape through enemies?