Source: Player's Handbook
You are proficient with tower shields
Prerequisite: Shield Proficiency
Benefit: You can use a tower shield and suffer only the standard penalties.
Special: Fighters automatically have Tower Shield Proficiency as a bonus feat. They need not select it.
Normal: A character who is using a shield with which she is not proficient takes the shield's armor check penalty on attack rolls and on all kill checks that involve moving, including Ride.