The great helm was a flat-topped cylinder of steel that completely covered the head and had only very small openings for the eyes and mouth. Later designs gained more of a curved design, particularly on the top, to deflect or lessen the impact of blows.
The style is sometimes referred to as a ‘crusader helmet’, but also as a ‘pot helm’, and a later variant with a more conical top is known as a ‘sugarloaf helm’. In Spanish they are called yelmo de Zaragoza, referring to Saragossa where they were introduced for the first time in the Iberian peninsula.
Although the great helm offered greater protection than previous helmets, such as the nasal helm and spangenhelm, it limited the wearer’s vision to some extent, and provided poor ventilation. A knight might wear the close-fitting steel skull cap known as a cervelliere, or its later development the bascinet beneath the great helm. A great helm may have also an attached mail collar, or camail, to protect the wearer’s neck, throat, and shoulders.
The bascinet evolved from its early skull cap form to supersede the great helm for combat. The great helm fell into disuse during the 15th century, however it was used commonly in tournaments where a version of the great helm, the a frog-mouthed tilting helm, evolved
http://en.wikipedia.org/wiki/Great_helm