Nice but...
All the levels are easy, but the game inmediately gets hard in the level with about 5 mines coming at you at the same time -that is a cheap way to make it more difficult. However, the game isn't as repetitive as one would think, because the levels are short, the enemy graphics differ a lot and the upgrade system is useful (although things are way too cheap). You could add more animation to the enemies. That's all... A passing mark would be a 7, and that's what this game is getting...