Way to easy...
it starts of challenging but as soon as you upgrade your weapons enough the game just becomes repetitive and boring i Got to about level 15-20 before I got bored anyways here's some suggestions:
lower the health of the centipedes because there pratically impossible to kill untill you get the 1000 credit gun
also you jump in power a lot when you do reach the 1000 credit gun as well maybhe insert a few more guns between 'cos it tok ages to get it and once I had it I was invincible.
Make the enemies faster I hardly ever got to use the vulcan because I killed them before i was en range in later levels
make levels shorter
make special bosses every 10 or so levels
after each level present a upgrade panel and have a "tree"
story cutscenes
vulcan has more ammo
vulcan can shoot backwards
have 2 use weapon 4 it to recharge, too easy alternating
that's about it