Really GREAT !
I don't understand why so many users are complaining about the difficulty...yes,it's a bit hard, but beating a level gives you a lot of satisfaction :) ! (I play in "normal" difficulty) .
Now, some complains :p :
1) There is something wrong about the game screen : some enemy units exits the screen (on right or left) but they still interact with you (exploding or firing) , in the third level there are only mines (but I can hit them with the lateral drones) and in the second level the tanks (same as before).
2) Is it normal that it seems that the boss in level 3 (yes, the one who runs up to you sooooooo fast :p ) seem not to be hurt by the last type of weapon (laser beam?) at level 5, the maximum ? The light of the weapon (very intense,but very COOL :p) covers the boss, and it seems to be unhurt after so many hits... I would like to know if it's normal (gotta improve my skills) or if there's something wrong (maybe the first one :p).
The shop system is GREAT,and the difficulty is well balanced (in my humble opinion) . Checkpoints would be fine, but I think that they're NOT essentials !
Keep up the good work !! :)