I'm sorry, but there's better and more concise ways to code much of what you have... Plus, this is very basic information on its own and there's textbooks which provide it much more clearly.
I'm assuming this is your first game? Basic necessities - upgrades, better physics, increase of challenge, music, etc. Not bad for your first time though, keep working at it.
Personally I really liked it. It's moderately good, very entertaining. You do need music, different kinds of enemies, faster rounds and more upgrades (gets boring from repetition quickly).