You're on the right track. Here a couple of things to improve:
> Add sound
> Don't immediately show the restart screen, just let me see why I died for a couple of seconds
> Reorder the levels, level 2 is really hard, level 3 is way to easy, level 4 has to many enemies
> Whenever you introduce a new enemy do this in a simple level, with just that enemy. So that the player can get use to it.
> Also add sound, but I already mentioned that