A nice game, with some defects
As other reviews pointed out, there are 8 sets of upgrades for air defense available (main gun included), while there are only 5 for the ground defense. If we include the number of levels available for each set, the air defense will surely be way stronger.
After playing two times and being slaughtered at no farther than lvl 12, I decided to go economic, using as little money as possible to upgrade as much as possible (with main gun as priority if same price occurs). That went fine, with five AAs (however it says six in the menu, so there must be an error while programming) at max power and the missiles too. Up to lvl 17 when I finally got overwhelmed I had everything maxed out except the main gun's LAST level of firing rate (and the most expensive, too).
It's not quite fun to see several things that just doesn't go right. For example, at level 17 or so, if the rocket soldiers gets hold of the chance to kneel down and start firing away, there is ALMOST NO chance of killing it because more soldiers keep flooding in to act as their "meat shield". In other words, the guns, especially since we are using the machine guns, should penetrate multiple targets while they can only focus on one for each bullet shot. Also, the air defense is so strong that the missiles appeared useless to me after having five maxed-out AAs. I originally bought the missiles thinking that it could also act on the soldiers, but it didn't.
All in all, it is a good defense game, but there are still a lot of space to improve.