I suggest that Tower Shields (50%) and multi-cast Tower(30%), have a 50% Pierce Reduction added to them.
If a boss has 40% pierce, the tower will now reduce the his pierce to 20%. The tower shield(50%) will still have a 30% resistance. The multicast tower(30%) will still give each player 15% resistance. Since everyone can train to tower or buy treasure cards.
This would help in PVP and PVE to protect a group or individual against a critical and a high pierce.
This would exclude School Shields only Tower and muti-cast Tower would do this.