Nice sound and graphics, apart from some sounds missing.
It would be nice to have some sort of particles when you kill an enemy, that way the player feels rewarded, because he just permanently blasted an enemy!
Also some sounds tend to get a little annoying when you hear them all the time, what you can do about that is go on your sound playing node, and randomize the pitch property, than every sound will sound a bit different and distinct!
BIG THING, i noticed a lot of lag when enemy bullets are spawned, i think you should store the preload in a variable so that you need to call preload() only once, this will make it so that it only lags the first time you ever call preload!
Very nice submission, great job!