The shield, as you said was the primary defencive weapon, but it was also a quite useful offencive weapon. Rushing with the shield, or hitting an opponents knee cap with the edge were common offencive uses for the tool. I am, however, suprised at how often I come across acounts of shields being no more than a minor set back when used as a defencive tool. I have read many times of shields being split in two, or being penitrated by a spear. I find when I am fighting with any thing other than a 14th century long sword, i like to use a viking style sword and sheild. But the shield should be used as much offencively as deffencively.