A good start
It's all in all a decent defence game. Its basic, but the whole thing is balanced, plus it was cool to see some innovation with the bosses as opposed to most games where it all comes down to just sheer firepower. If you plan on making anymore of these in the future, a few pointers for you:
1)Graphics are hugely important for a game like this. If nothing else, a bit more detail to the pathway and background make for a more visually appealing game. After all, your only looking at one scene for a long time, so it pays to make it cool.
2)An addition of other maps of increasing difficulty, or even a difficulty setting that would make enemies more powerful, or give you less points per kill are relatively easy ways to add some replay value to your game.
3)Creativity. Remember, these games run rampant with defense games, so you need something to throw people off. I loved the special bosses in this, so stuff like that is always great to see.
Good luck, and hope to see more of your work.