Thank you very much you all =D
@ramkey : Killing enemies offscreen was "intended" in a sense, they should probably spawn a bit closer but that was intended ^^.
@jason-wilson : The "level display" was, with the background moving down, my way to say "look, I've seen the theme, I know I'm not going to be in it, but I've tried". I went into this LD really wanting to make a Super Fantasy Zone clone, that's a shoot them up where you need to kill special enemies to go to the boss and where the levels are big horizontal loops where you can go left or right, hence why this feature is there. Obviously, when I saw the theme, I became very sad and decided to just go with some visual stuff but still do what I wanted to do, I think it kinda paid off in the end ^^.
The blue cube is one of the two attacks of the "boss" (the big wheel thing), there's a laser that stops and then goes super fast and there's the slow cube. To me, because I've seen it in other shmups, it made sense as a "bullet" but now that you say it... it does look like a power up ! Sorry about that. Should have made them destructible as well, that would have worked and that's a very good idea... I should have had but didn't ! ^^
@noef : The camera and sfx issues are real issues, the speed parameter is a consequence of a dropped feature : the game was supposed to have a shop and in there, you would have been able to buy speed upgrade and damage upgrade. I didn't have the time to code it sadly, and so I tried to put a base speed value that would "work" to me, but didn't have the time to really solve this either.
@bitshift : ^^. ZQSD is just WASD in the AZERTY layout (which is the one I'm using, as it's the standard in France), using it on a QWERTY keyboard must be a weird experience !