European Kettle Helm
The kettle helm was the staple of medieval headgear. Worn primarily by footmen and gaurdsmen in the 1100s, this brimmed metal helmet ("Chapel de Fers" or "iron hats" as the French called them)provided adequate protection while allowing excellent vision and mobility. They were also fairly light and could be worn for long periods without incurring fatique. There's also some anecdotal evidence that soldiers would actually cook stew iinside the helmet when they had no pots and pans. Not sure if it's true or not, but seems plausible enough.
It was strongest against attacks from above, like, say from sociopathic knights on horseback swinging swords or warhammers. Or, perhaps, from sadistic archers on castle walls firing arrows or throwing rocks down onto invading soldiers. It was also pretty good against high, overhand swings of sword or axe.
Kettlehelms were much easier and less expensive to make than more comprehensive helmets, but provided excellent protection for the money. This made it ideal for soldiers and gaurds, who often had to be outfitted in large numbers. Pikemen are often seen wearing this type of helmet as well. And because of the comfort and vision that these provided, even some knights would wear them to battle.
See Strongblade's European Kettle Helm