Mudflow, Common, 3b
Nature
Enemy tower: -3N
Enemy Wall: -3N
Enemy bricks: +3N
N is the # of Nature cards in hand
As it is now, this is a common card that only costs three bricks and will always decrease the opponent's tower by 3 with the potential to decrease it by 24. Ignoring wall attack and brick gifting, the tower reduction alone calls for a limitation.
Possible fixes to this are capping N, having N only start counting after the third or fourth Nature, or making the tower reduction N rather than 3N. I'd argue for the last option myself.