Too much of the same
After playing for about ten minutes it got boring, since it was all the same things over and over.
I spent ALL of my missiles and mines(and more, probably 60 +30 total+ hella lot of normal bullets) on a single medium size enemy unit and it still wouldn´t die(using a small unit), after about, lets see, maybe 15 minutes of continously firing at it it would not fire back at me anymore, does that mena it died? My guess is that it is immortal, but you wouldn´t tell us that as far as I know.
To improve: Lower the destroyers health(or make it kill-able), enemy life bars when they get hit(especially bigger units), increase the power of both the mines and missiles. Give an option of several soundtracks for the music, since this is a game that is likely to take some time and the music will get too repetetive. More weapon types, like you get to choose between some types. The ability to come back after you die, this would require less health for yourself though. Better AI, online multiplayer would be sweet as an alternative.
Total score: 5/10 4/5