Oh my god, where did the time go? I think I literally just spent an hour playing this game. Honestly, I have played many, many games in this competition so far and... I think this one may have been my favorite. I seriously did not give 5 stars overall to many games - maybe one or two. None have been more deserving than this.
In my opinion, the most important thing a game should have is satisfying, engaging gameplay - check. The shooting is fun, the movement is nice and responsive, the enemies are appropriately difficult (save for the boss - a little time in between shooting waves would be nice, as currently the player is always trapped in a corner).
Additionally, a good game must create an interesting atmosphere, either by the use of effective visuals or sounds - check. I love the art style; simple as it is, it still feels so polished, and I love the design of every enemy, tile, projectile, and upgrade in this game. The lighting effect looks really neat when the power goes out too! The sound design is great too; the sounds were sharp and subtle, almost melodic (my sister heard the sound of continual shooting at a high fire-rate and thought that it was actually a song).
I don't know why such a simple game captured my attention for so long... Maybe it was the feeling of progression as I earned more upgrades, or the variety of situations and rooms I would have to survive. Seriously, great job.