Heavy Cannonball Shard bonus damage seems not right.
When max, the bonus damage is 400% of Cannon ball tower's damage.
My tower attack damage is 98542. if 4x of this damage. it should be 394168.
On the other hand, my defense power is 7774. if 4x, it's 31096.
In the game, tested on dummy. the rolling cannon ball's damage is 32685.
So either the description of the shard is wrong (indicating 400% of defense pwoer, not tower's attack damage) or the damage is really low (plus the rolling cannon ball can't do critical damage), not worthy.