Good, but not hard enough...
I kept playing, thinking it would get harder, but it never did. I think one simple way to make this harder would be that the highest level tank should be impossible to destroy with the sniper rifle. The only way to destroy it would be with the mortars, artillery, airstrikes, the big bomb, and of course, the turrets and cannons at the end. Then make the 2nd highest tank be twice as hard as it currently is. Also, increase the number of soldiers coming through so that it becomes almost impossible to pick them all off with the rifle. I played through the whole campaign and pretty much the whole time my aim was hugging the left side of the screen, just picking off everything that came through... only 3 times did it ever seem like it was getting outta of control and they were bunching up at the front of the base, but even then, there was no sense of worry or panic that my base wouldn't hold up. The base health never went down below 75% the whole game. Maybe another way to make it harder would be to make the sniper rifle upgrade take a little more EXP to upgrade. Still, very fun game for me, just hoping it would've gotten a little more frantic. I really did feel "godlike" with my rifle... and that probably means it's too easy.