Just finished
An excellent shooter! There could have been more levels and I'd have loved it even more. As it was, there was plenty to do.
I got past the Level 4's, but the secret is to tackle them in the right order. You need to unlock one aircraft to have a hope of getting the other one.
The "escape" option is a sort of "checkpoint" for people. At least it keeps your credits. Very useful when trying to upgrade!