Good concept, poor execution
I liked the idea of building your own maze, that was a very original take on defence games. I also liked how the price increases for the units as time goes on, however these increases seem to be too low to make much difference and as the upgrade price was always 10 building was extremely cheap. The graphics were okay, no complaints there.
However the execution of the game is very poor. You were missing a lot of features that we've come to expect in defence games (perhaps telling us what the next wave would be, how many waves left, being able to call in the next wave early, speeding up the game, viewing units strengths etc). The units were unbalanced and there seemed to be very little skill in where they're placed. The units didn't really have any special abilities so as long as you placed them so they could hit the track it didn't really matter. The enemy units were far too weak. Even the mothership thing would get shot down very quickly. At one point I didn't build a new unit for over 20 rounds!
As mentioned earlier I liked how you could build your own maze, but perhaps you could have counterbalanced it so that the longer you built the maze the tougher the enemies became. I didn't want to build a complex maze because I felt that it would be "cheating".
Basically this game could have been a lot better if a bit more time had been spent on it, a few more units produced and a little more thinking about the dynamics. I'm sorry if this sounds harsh, I know you've probably spent a long time on this, I'm just trying to provide some constructive criticism.