great potential
It was a bit short, I am glad that someone finally game out with a game like this.
I agree with a few of the others this would be much better with line of fire indicators, personal upgrades, troop upgrades, and trench upgrades. I also think a higher unit cap may be a bit better (maybe like 5 more units).
Another interesting idea would be to lay down traps or mines. Finally having an "Endless War" setting in addition to survival may be useful if people want to see the upgrades (if you add them) to their full potential.
Lastly a difficulty adjustment may be fun for some users (I found it to be easy)/