I liked the mechanic of switching between different small rooms. It was very well-put together. It started very harshly, no title or even a key press. Also, is there an ending? Or does it just go until you run out of lives? The different weapons were cool btw.
I would recommend doing things in a different order. Some important stuff (like a menu or end screen) was missing while some less important stuff (cool and different weapons) were there.