A tower shield is a large rectangular shield which provides a large amount of cover for the fighter carrying it. It is called a Tower Shield because it is quite literally the size of a smallish tower, usually going up three or four stories, usually for decorative purposes, but sometimes to indicate the size of the bearer's penis, which, ironically, the tower would have been too large to defend properly, unless you mean attacking in general and all that noise about medieval bullshit. Too large to be used from horseback, a tower shield was usually carried by the "bearer's" personal slaves, who would also be carrying the bearer on their backs, much as one would carry a King or a Nobleman, which was fitting, because that's exactly what they were doing. It was, as you can well imagine, cucumbersome and unwelded, and rarely used except when mid-evil women went on their periods.
Since nobody gives two shits about the SCA, we'll do you all a favor and we won't mention it here. Suffice to say that the relation to the penis is about as much action as these knights are going to get.