I assume you're going to include powerups to do something with the gold collected. I see there is some variation with enemies, but would recommend faster progression to avoid it becoming tedious.
Two things about the game as it stands:
- Personally, I find it becomes annoying fast if I have to keep clicking to fire. You might want to consider continuous fire.
- I've stopped playing for some minutes now, the game is still running, lots of enemies on screen and I haven't died yet. This suggests the amount of lives/health can be toned town a bit. This depends in part on whether you're planning to do separate levels or not, and what happens when lives run out (continue/game over?).
Finally, there are a lot of games like this around. This means that (1) though you create a decent game, there will most likely be a better executed version of the concept available; (2) people (myself included) might not be so enthusiastic about the game when it includes only stuff that has all been done before. So: try to come up with an original feature that makes your game stand out from the rest.