Too slow paced after level 15
The balance isn't that great, but it's a fairly solid game already. Despite what the other players say, I found the game to be not challenging at all, and I liked the money system except for the miners, who get wasted too quickly to have value.
My biggest pet peeve: after level 15 or so, the waves are just too freaking long. Level 20 lasted at LEAST 20 minutes. I played until level 22, then I just quit because I was starting to doze off during waves.
My strategy: 4 gun turrets up front, could be replaced constantly, 4 huge mortars, to take out the mortar aliens, 3 generators (plus the one you start with, totaling 4), 13 mortar troopers just outside the range that the mortar troopers hit when they fired on the gun turrets.
As long as you replace things as they fall, you can't lose. Don't bother healing; it's more expensive than building a new tower. Money is no issue this way, as long as you build up slowly but steadily. The money cap at 1000 is kind of irksome, but doesn't really negatively impact the game.
I would recommend you take the emphasis off of mortars. The entire game was played through using solely mortars and I had no reason to switch it around. Any trooper using a gun was instantly scrapped by the enemy mortar fire, making them useless. It seemed that there were only two types of enemy troops: "mortar" and "I don't give a shit, this guy can't hit me because my mortars will scrap him"
Have you considered fast enemy troops? Something that would move just too fast for the mortars to hit would require the player to build up their gun strength. Also, try to think of a way to fix the fact that every type of human unit is useless except for the mortar once the enemy mortars roll out.
What the hell does the $300 tesla coil do? I had one for a few minutes, it did nothing, and I eventually just ion cannoned it to death in order to make space for a normal gun turret. If I were you, I would cut that out completely and instead i would add some kind of healing station or shield generator for infantry.
Finally, the support stuff is totally useless. Healing doesn't heal enough, and only the ion cannon packs enough punch to make it worth using every once in a while, when your mortars are seeming particularly inept.
While I'm here, I figured I'd point out a bug you have. When you click to place a turret or a unit, a blue circle appears, but your unit count does not yet go up. While that is in place, you can make more units, even exceeding the limit, until the units finally appear and reach the limit. it's possible to have all of the craters filled in this way, and possible to have 2 or 3 men above your maximum by quickly adding units before the blue circle turns into your unit. You could fix this by making the unit/building count go up when the blue circle appears, instead of having to wait for the unit to appear completely.