Great job making this in such a short amount of time! Your pixel art is really nice and reads well, with a good sense of depth and perspective. The enemy's pathfinding is a little wonky, especially when you place walls next to each other. I found it a little frustrating when my shots would bounce a foe out of range of further shots, especially in the northernmost section.
Overall, though, you did really well. I hope you develop this game further, because I'd love to play this for hours once it has some more polish!