Bullet Room

First of all, I'd switch X and Z. Most games I play have them in reverse for jump and shoot, but maybe that's just me? I don't know. 

I enjoyed this but it was a little too simple. While the game builds up and ramps up with new enemies and patterns, aside from that there isn't much buildup or enough changes to keep it fun and exciting. I'd recommend checking out Super Crate Box and Woah Dave! to see how they created simple one-screen games that keep building momentum and add new ideas and mechanics to keep things fresh and engaging.  

Woah Dave! For example, has the environment shrinking and shifting. Different enemies show up the higher your score, not just after you clear a wave so it only gets harder the better you get. This game could use a lot more juice and effects. 

Still pretty fun though. Allowing us to hop on enemies is a stroke of simple genius and I love it.