Needs more.
It's a good game, but it is really lacking. There are a lot of things you can add to this to make it better.
First, sound effects. You should have SFX for everything in a game.
You should have a sound for when you shoot your gun, and one for when the seagulls poo.
Second, music. I would recommend you add some kind of background music. You could have easily put in the Space Invaders theme in here, or you could have found something better in the Audio Portal.
It's a good game, but it just needs some improvement.