The shield; oft whined about, oft under-rated, and mostly ignored as something that is uncool. In DnD/d20 the shield can reduce your chance of being struck by 5% for a buckler to 20% for what that game dubs a tower shield. Unfortunately that’s not a the only thing a shield does. It might just be the most obvious one to an external observer.
Those of us who took our asses to the real side of life and found out what the weapons were, read history, and checked our facts in martial sciences realised a shield is a weapon. It’s not a lethal weapon, per se, but it is a damned fine complimentary weapon. In short the shield aids a character’s ability to melee.
Many systems separate the skill of attacking from the ability to defend; or even make defense something that is based on pure natural ability and hardware with nothing to do with skill at all. This is not the way it works. The two, attack and defense, are inextricably tied together.
Modelling this in an easy way was elegantly done in Simple 2d6. The skill for Melee covers attack and defense simultaneously (see this post). From there it was very easy to model shields as aiding the fighter’s combat ability with a bonus to Melee. With larger more robust shields the wielder also gets a bonus to their armour value. So simple it’s beautiful.