A Few Problems
I like the concept of the game, but there are major flaws. First of all, units are very expensive, I cannot build much at the beginning of the game, Secondly, the cash gained for killing hostile units is not nearly enough considering that enemies march down the road quickly. At least you should give players time to upgrade their defense between waves. Thirdly and lastly, there should be more units that players can choose to build such as tanks, castles and missile launchers. I strongly advice you to take a look at "Pokemon Tower Defense" and learn how to make a good tower defense game. You have potential, but there is still a long way ahead before you become a good game designer.