Mobile Fortress (Combat)Source Armor Master's Handbook pg. 12
You use your tower shield to defend yourself or an ally at a moment’s notice.
Prerequisites: Str 15, Mobile Bulwark Style, Shield Focus, Tower Shield Proficiency, base attack bonus +4.
Benefit: While using a tower shield, you add half of the shield’s bonus to AC (including its enhancement bonus) to your touch AC and take only a –1 penalty on attack rolls because of the shield’s encumbrance. While using Mobile Bulwark Style, whenever you would use a tower shield to gain total cover, you also grant any adjacent ally total cover against attacks passing through the selected edge of your space.