I would alomst prefer a more 8-bit look to show the retrostyle of the game, but the graphics are ok the way they are. Gameplay is fluent, complexity factor is rising slow, but it is rising the longer we play.

You should consider to add more content if you want a higher replay-value: Highscores (so players can compete + fits to the retro-atmosphere), Medals or Upgrades (even if they weren't common back in the days). Also, you might add an autofire-function (one button to turn it on or off) so gamers don't have to spam the space key.

Nice little homage to the arcade-era anyway!