I think Necrophobia did a much better job with this kind of gameplay. The difference is that there you could move units directly with your mouse. I found having to use arrows to pull units to a reserve and then directly back out to be frustratingly clunky and tedious.
While you can skip the intro, I otherwise agree with Talanic. I fired the same projectiles at the same enemies on the same background until my score said I had killed 250 of them. It was at that point I notice the lack of a pause button or a point to continuing to play. 1/5.
I think the game has a decent core. The problem is that there's very little strategy involved. You simply create an appropriate design (I staggered placement in each row) and then upgrade your way to victory. The game needs more tower types, and time to build in between waves.