Overall quite a fun game! Which can even be ported to mobile phones. I'd like to mention some of the aspects I found amusing and interesting:
First, the gameplay is really easy to understand. The movements are smooth, and the sound design was excellent.
Although I wished that I'd had a difficult and more randomized difficulty of opponents. This comes from the fact that, as others also mentioned, the bullet speed is constant and only the number of opponents change. But they only shoot the bullet, no matter where they are, basically not in the sight of the player. Although this might seem like a feature, it reduces the fun that can be taken when facing those opponents.
What I want to say is that probably a better AI for opponents would make the game much more fun than it is right now.
Also, you can probably change the map whenever the player reaches a specific level. or maybe, allow the player to gather points, healths and armour from within the arena.
Again, overall a good game and a fun one. Wish I can also see it in app stores.
Creativity: 4/5
Use of Themes: 4/5
Gameplay: 3/5