Great action game, but becomes easy and repetitive
First off, I really like this game. It's very entertaining and fast paced but unfortunately once you progress, the action slows down and it becomes a game of patience.
Once you get all the upgrades the game gets way too easy (to a point where you can actually let go of the mouse and the automatic guns do the rest).
It's not until around level 50 that the enemy gets strong enough so that the player has to start shooting the planes manually again.
The game gets progressively harder though, but that happens way too slow. From level 80 and on I found myself holding the defenses quite well but slowly started losing resources mostly when I lost concentration due to the repetitiveness.
I ultimately got to level 102, at which point the screen was constantly filled with planes and I could only kill the really slow ones (shooting them about 30 times to kill them). I couldnt build any tanks anymore cause they got killed immediately by the planes so I had to resort to using the shield on my base in order to survive.
It was a really slow process for the enemy to finally destroy my base (1 1/2 hours!) and pretty anti climatic once I died at lvl102 to see that I "failed" my mission :)
Other then that, great game.