728 is the score I got.
Some things I have to praise;
the ship movement is good
The moving down pattern isn't as good as the classic space invader but better than a lot of other space invader clones on here.
I like the fact your gun is used for both offense and defense.
The bad;
Having to spam the shots on the keyboard which if you were worried about having them hold down the space bar trivializing the challenge; just add an ammo system next time.
Lack of sound design.