The game is fun, and I like the fact that we can create our own boss to fight against. Just found a problem that after a level, the main menu and the next level button are not working. I will give 4/5.
Your game is very nice. I only would add a level selection with a star rating for each level.
Just a question, I thought that would be a boss on last level, because you said "giant bosses" on description, but I just found one(The giant statue).
I'm giving 4/5
Your game is be very nice for kids, because it's a cool way to test math. I found a bug, after getting a correct answer, some times, I got a wrong and lose some time. I'm giving 3/5(For kids)
Well, first I would add a menu with level selection, then a power meter to the cannon. You could change the background too, and add some objects that would affect your cannonball. This would improve the the game some kind. I'm giving 2/5 right now.
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Thanks for reporting that bug about the buttons -- we've fixed and re-uploaded.