Nice idea, but it could do with a bit of work.
You should have one massive robot on the final level. Your ship would have to shoot at the rockets he fires at you, and try to disable each of the cannons on him. After that, the robot's head would start shooting lasers at you (which can only be avoided, not shot down). To finally win the game, you'd have to blow up the robot's head.
There should be a background of the destination you're trying to get to. On each level, it would be closer (or just steadily become closer during each of those levels). By the final level, it would be absolutely massive and extremely near.
Give the graphics a bit more work, add a bit more detail. And make small, subtle animations on the ships, such as a tiny radar spinning around.
The only sounds you have are 3 tracks of music. Not good. Make sounds when you're shooting, or when something blows up.
Speaking of blowing up, you ought to make the explosions animated as well. Maybe just expanding and losing alpha?