Really nice game! It was very clear what to do, and the controls were very simple as well. I liked that you decided against having a separate button to shoot. The way you did it is nice and easy. What I thought was really good is that the hitboxes of the enemies were nice and big. I often felt like 'oh, I thought that would miss. Nice!' when I hit, which is a lot better than 'but that should have hit!' if you just miss an enemy.
Maybe the game could become a bit more challenging a bit faster, because I got a little bored having to play through the easy part before it became harder. But this might just be me of course! ^_^
All-in-all great entry and amazing job in only three and a half hours! I enjoyed it :D