ok. its pretty goring tbh

It works, but it gets kinda boring due to there not being much going on most of the time. I'd recommend increasing the cannon's rate of fire and adding more aliens. Also make the aliens appear from all directions instead of just the 4 cardinal ones. Making some of the aliens move at different speeds would also help making the game more exiting.

Then again, I can see that you are still just experimenting with unity at this point so honestly, it's just nice to see you try different things.