Whew... This really needs improving.
Don't get me wrong... this is actually an OK game; however, there are some serious issues that shouldn't go unchecked.
First of all, the gameplay is repetitive. I see the same units charging at you all the time, maybe 3 or 4 kinds. You should insert more variety in opposing units, like trebuchets and catapults, more designs for infantry, and maybe opposing calvary.
Second, the draggin of the units onto the field, really sucked. You should try and make it so that when you click on the unit, it comes onto the field itself.
Third, the upgrades seem ineffective.
Also, the units are really weak. Upgrading their lives did nothing, in fact they died quicker than usual.
Lastly, try and improve the unit graphics. I don't know if you were aiming for stick figures, but it doesn't really suit this game.
Overall, this game wasn't exactly worth my time, but with some new towers and units, and the above mentioned stuff, it could be a lot better.
Keep Trying!