You can use your tower shield to block magical attacks.
Benefit(s): While using a tower shield, you do not suffer a penalty on attack rolls because of the shield’s encumbrance.
While using Mobile Bulwark Style, whenever you use a tower shield to gain total cover, you also gain partial cover (and grant partial cover to adjacent allies) against spells passing through the selected edge of your space. Also, while using this style, you can gain total cover against a single attack made against you as an immediate or swift action.
Pathfinder Player Companion: Armor Master’s Handbook © 2016, Paizo Inc.; Authors: Alexander Augunas, Robert Brookes, Anthony Li, Luis Loza, and David Schwartz.