Not very good, probably just a lack of experience.
Don't bother comparing this to a decent flash game, anyone. There's no point.
Here are some things you should do:
1. Add a short tutorial (with a skip option) at the start, right after you click "play".
2. Try putting the cannon in the centre, it's annoying as hell to hit stuff on the right, it's REALLY slow.
3. Try making the cannon go faster at the start, it's annoying. Or you could make the explosion bigger, to compensate. But the whole "RPG" thing never works that great if it's annoying at the start.
4. Make it so the cannon doesn't fire when the upgrade menu is up.
5. Likewise, make the normal menu pause.
6. Add some interesting features, and generally clean up. The game should be interesting for the first 30 seconds, absolute minimum. Sometimes players will be willing to wade through the boring stuff if there's promise of something interesting, but they won't like it. And if there's no promise of something interesting later on (like in this case, it didn't seem to have any indication that it would get much more interesting), you can expect them to immediately scroll down, give it a 0/10 review, then close the tab and go play something else.
