Pretty good, with good innovations
I'm a big tower defense lover and try to play as many as I can get my hands on. I was kind of sketchy about the HAVING to use the number keys to buy units, you should make it so you can choose, but after a while you do get used to it. I enjoyed the airlift feature, but I think you should have made it so you didn't really have to use it almost right away.
A couple of things I think you should fix are:
1) The x on the boxes that appear between levels makes it seem that you can close those boxes, but clicking the x doesn't do so.
2) If you have guys placed near the critter exit they continue to shoot at ones that have already passed into the civilian zone and that can severely effect your defense, I lost more then a few security points from that.
3) I didn't find the third piercing unit to be that useful, it's rate of fire and damage seemed to be offset TOO much by the range and cost of it.
Overall I think it has excellent potential.