I dig the environment/tile art, it's got a nifty look to it.
The enemy designs made for a nice challenge. The game teaches you pretty early on to take it a little slowly and carefully, and it definitely lives up to that. I also really like the use of the B button to change facing direction.
This style of gunplay could benefit from level design that forces a little more engagement. Most enemies in the first level are either in rooms that can be skipped entirely, or are facing away and can be killed easily. I'd love to see layouts that force me to really consider my approach. There's probably a lot of cool stuff that can be done with this format. Ammo, cover, explosive or destructible objects, etc. I'm eager to see where it goes.