I like the pixel arts and how the tutorials on the walls and floors.

One thing is the bullet and the scene blends in together a bit and was  hard to see where I was shooting.

Love the graphics and  game loop! The only tip I can give is to make a sound for shooting. Either way it's awesome you were able to make it in such a short time.

It looks good, but in my opinion you have focused a too much on that. 

Most importantly it's extremely short. I think it would have been a much better idea to have a scoring system or an other way to make it last longer. For an next time start the focus on gameplay. :)