Quite enjoyable, but there's room for improvement.
The game is easy to understand and play, fairly addictive once you get the momentum going. In my opinion, this game is actually very easy to play if you are used to playing such match-3 puzzle games, but it can be quite challenging to those who are not as such and it can be hard for them to beat the later stages. Also, the game is too short for most of the level three upgrades to be worthwhile, so i suggest either making more levels or lowering the unit upgrade prices. What I suggest to add in to this game if you're planning on a sequel would be to:
- optional difficulty settings at the start
- improving on the graphics of the soldiers
- modifying the tile replacement speed (It's more significantly slower to clear a vertical line than a horizontal line)
- perhaps adding a casual mode where the player can play on infinitely until he is overwhelmed by the enemy
But overall, I gave this game an 8 because it's fun to play. Great Job and I hope my review has been constructive. :D