Half the points
I gave you half the points for being half way there. You have a good premise that could lead to a pretty good game, but there are some major balance issues which I'll point out in ordered list :P
1. Upgrades - the cost of upgrades needs to increase as the level of the upgrade increases.
2. Enemy difficulty - The enemies were fairly difficult early in the game, but after a couple of upgrades, they never got near me. The enemies need to improve in hitpoints as the game goes on. Also, some more interesting enemies would make for a better gaming experience.
3. Towers - The laser towers are far too strong, especially for their price. Also, as mentioned in other reviews, the detection is off.
This should be enough to keep you busy. Balance is the MOST important factor in defense games. Keep up the good work.