A solid game
This review is based on my experience of the game. If the experience of the game changes in levels later than I have played, then please take this into consideration when reading my review.
What stands out most for me is how driven I find myself to learn from the last run and play again, so this game certainly excels in replay value. Even in its simplicity, the diversity of each play certainly makes it an intriguing experience.
However, it is this simplicity that draws on some of its value as well. I would like to see some more diversity in the units, other than minute color and detail changes. Perhaps some diversity in the direction they come from would be a good way to challenge players in later levels, as opposed to increasing the defense of enemy units. I would also like to see a brag board to post our scores on.
All in all, it is a solid game with flaws that do little to mar a well polished surface. A sequel with a bit more polish and diversity would be certainly be welcomed.