The game is very good, engaging, funny and also very appropriate for the theme. The physics based gameplay is the cherry on the cake.
Here are various thoughts after playing your game: some animations for the dragon would make it even more addictive. As already mentioned by @phillip-scott the aim felt kind of strange also for me (it is probably the fact that the mouse gets locked at a certain radius). A couple of times that the fireball didn't explode but stayed on the ground for some time. Maybe this is a not a bug but I thought that it could be useful for you to know.
At last I think that it would be good to add a background (or something else like particles) because if you are above all the castles and only the solid blue sky is visible you don't have any way to know if you are moving or not.