Alright for a side scroller but the enemy rounds passing through the walls is a detriment for me.
Fun game, not to hard and not to easy.. I didn't beat it yet but getting close.. I will make sure to eliminate all droids, even if my life depends on it.. (video game life)
Maybe you could let the play once they beat the game start over with the total points to spend on upgrades and have them start the score after buying everything that they can over to zero