Wow, I love all about this game. Can't even begin to fathom how you did it all in Construct (I just used it recently and is still hard for me!) Great. I'd buy it.
Two things though: Z is on certain Keyboards far away from x, so controlls can be tough on the keyboard. Jump on space would maybe fit better, as it is one of the main actions it should occupy a prominent key. But with a joypad attached it was nice.
Second: The cannon sounds can be LOUD, particularly in the second part, where there are 3 shooting sounds together, I barely heard the music anymore.