I spent hours on this game, on the 3rd to last mission the difficulty got ridiculous and I was just spending the time pressing Z, repeatedly to rebuild my destroyed units, I managed to win.
The on the 2nd to last mission it was the same.
I strong suggest you don't play this game, you'll spend hours until you get to the point where there is no point to continue and you get no payoff for all of your time.
Rated 1/5 it was a really good game until level 12, when the programmer decided to ramp up the difficulty to an absurd level. It's lazy programming make one level tricky, then the next almost impossible, hence my rating.
Very good game, the only problem is upgrading your turrets. I researched to level 10 on all of them, but was unable to upgrade past level 3 because of the expense.
It would also be quite good if you had an energy level for the main cannon, so you could save up your energy and use the plasma cannon for a longer shot than normal.