very basic. you should add a weapon store aswell as the upgrades, and mabe purchase troops or auto turrets. also the levels should be more original and the map is to small, the enemies shoot you almost as soon as they are on the screen. due to lack of originality and replayability 2/5
for those who didnt realize space button PAUSES THE GAME!!! very easy knowing this. just put bombs at the bases of the bridges, wait till all enemys are on the bridge, press space, click all bombs and press space again. bye bye army guys!!