A tower shield is a large rectangular shield which provides a large amount of cover for the fighter carrying it. Too large to be used from horseback, a tower shield was a carried by a foot soldier. It was cumbersome and unwieldy, and rarely used in the medieval period.
In the SCA tower shields are not used in tournament combat but are allowed at wars. It is for this reason that they are sometimes affectionately referred to as war doors and are really only useful in a shieldwall.