Simply amazing! This truly feels like a demo for a full game! You could definitely release to steam as an early access and people would dig it. Seriously!
The fast paced movement, the way you glide through the level, shooting and getting upgrades, is super enjoyable. There many different enemies with different behaviors, making it mechanically really interesting. Instead of just having one or two enemies that boil down to small & weak and big & strong , there are ranged units, units that jump trying to ambush you, others that explode, wow!
The art style of the game looks amazing. I'm impressed by what you have made. It looks super clean and you managed to create something unique and with personality. The music goes really well with the game as well. I love how it's all calm when the gates are closed, but as soon as you open them, the beat starts playing again. It matches so well the feel of the game!
Honestly, the only thing I'd like to top it all off, the cherry on top of the cake, would be a boss type enemy. You should definitely consider that if you keep designing this game.
Honestly, and again, simply amazing! Easily my favorite so far. Definitely give us a shout if you expand on it and make a full release!