A simple twinstick shooter not really doing anything different. I've played this game before. Hell, I've *made* this game before. (It flopped. Probably because it wasn't doing anything different.)
So. I'll just list some random features that don't exactly distinguish Space Guardian, so much as tell you what bucket of twinstick shmups you've played before this fits into.
Mouse control. Smooth movement. Colliding with enemies doesn't hurt you, just their shots. Abstract pixel spaceship. TONS of screen shake. Scrolling, but confined to a small arena. You can't shoot down the enemy's slow homing missiles. Wave-based enemy spawning, but no real cutoff between "boss fights" and regular fights.
I just realized I could have said "Geometry Wars" and you'd probably have gotten the gist of it.
Recommended if the things I said made you happy.
Oh, and there's a TON of powerups that spawn at the same time, but you can only use one at a time and the player movement is floaty enough that you'll probably collect some without meaning to. It definitely does NOT stack the powerups you collect into a ridiculous arsenal of extra guns all firing at the same time like some sort of absurd bullet katamari, which would have been the awesome thing to do with that many extra powerups. I'm really not sure what the author intended, so it's possible there's a layer of gameplay I didn't understand, here.