I appreciate how you added upgrading weapons as a mechanic, and added sutile story in the level selection (the UFOs in the above the cave).
But the problem I see is the level design, because the main difference among them is the enemy quantity. In order to add more interesting difficulty, I think you should design levels thinking in making the player create more interesting strategies than just clicking the mouse constantly until the level is done.
Also, there is no reward for winning a level, or punishment for losing, apart of getting coins for your upgrades, but thats independent of winning or losing.
But after all, I think it is a good game. I saw this was your first game, and I think its great for a first try!
Good luck! :D