"The Tower Shield is a massive full length tower shield that offers supreme protection but is heavy and may restrict the reaction time of the wielder."
This is the defensive offhand is best used in coordination with your teammates, as it often becomes the front of a push into enemy lines. From arrows to axes, the Tower Shield's high coverage makes most enemy attacks a non-threat so long as you can guard consistantly, and the stamina it takes to guard is almost laughable except against the mightiest of blows. However, there is a short prep time before guarding, so don't be too hasty to put down your shield unless you're certain it'll benefit you.
When placed on your back, all shields block projectile damage, but not melee damage.
With that in mind it is probably the best shield to have on your back for the best passive projectile protection. Due to the fact that it covers a superior area of your back whilst not in use.
It is strongly recommended to use a third person perspective while using a Tower Shield, as the first-person blocking animation almost completely covers your screen. Alternatively, the Kite Shield may be favorable for first-person perspective players for its greater visibility.