i see this as op in my mind. if you sacrifice a creature to get its cost back in pillars, (assuming your mark is the same) then on the first turn alone you have already out gained the creature's cost, if you dont have the same mark, then it would only take two turns to DOUBLE what the creature was worth, i can't see this as much of a fix but put a cost/2 effect on it or it depletes 1 of your towers ea turn, but that seems to be too much nerf. i just see it op as it is