In Piecemeal helmets are extremely useful, but they do not provide anything akin to an "armour class" bonus. Rather a helmet helps prevent critical hits.
Regardless of ones armouring or lack thereof, a critical hit is scored when the attack roll is either a 20, or beats the defender by 5 or more. This a skilled warrior is more likely to score a critical hit when facing a lackey. Helmets increase the range required (up to needing to beat the defender by 10 or more) at a penalty against awareness checks and detection rolls.
This is not new and works pretty well. What I have decided to do is streamline the helmet types available. Before I had fallen into the old trap of too much specific information, and tying it into historical helm types. There was the leather cap, the mail coif, the kettle helm, the open helm, the great helm, the beaked helm etc etc etc. This level of detail is not really needed, and it still discounts fantasy variants that never existed in real life (nor would they in any environment where a medieval warrior DIDN'T have to worry about air attacks and flamethrowers).
Thus it's been toned down to "Cap, Helmet/Coif, and Great Helm". Which seem to be the only variants people used anyways. This makes it a little easier to quickly generate creatures on the fly too ("does a dragon's thick skull count as a kettle helm or a mail coif?").
Welcome to your doom
8 hours ago