It looks great, the stretch and squash on the cannon with the huge plume of fire make it fun to shoot. It doesn't feel like there is a lot of strategy to the game and I'm unsure how to play better. Despite that I feel very proud of my high score of 45. I'd like to see more elements indicating how I'm doing like an indication of how long before the humans have before the explode, I think that would add more decision making opportunities. I also liked the UI you've created, the high score tracker makes it all the more tempting to have one more go to try and beat your best score.
Good Job!