The great helm or heaume, also called pot helm and barrel helm, of the High Middle Ages arose in the late 12th century in the context of the crusades and remained in use until the 14th century. They were used by knights and heavy infantry in most European armies.
In its simplest form, 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, as verified in historical re-enactment combat.
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, so was generally put on only just before engaging. A knight might wear the close-fitting steel skull cap known as a cervelliere, or its later development the bascinet normally, and the great helm over this. Contrary to what is often claimed, great helms are not overtly heavy, cumbersome or uncomfortable. Modern re-enactment great helms, which are made from thicker steel than the medieval examples for safety reasons, weigh 1.5 to 3 kg. While the visor slits are usually only some 20-30 mm wide, they do not restrict the field of vision to great extent since they are very close to wearer's eyes. A great helm may have also an attached mail collar called camail to protect neck, throat and shoulders.
The bascinet evolved from its early skull cap form to supersede the great helm for combat, generally with a hinged faceplate. With the evolution of helmets and armour, the great helm was superseded generally in the 15th century, being used thereafter only in tournaments.
The great helm is today especially popular amongst live-action role players and in medieval re-enactment such as the SCA. It is inexpensive, easy to manufacture with even rudimentary equipment (metal scissors, drill, rudimentary anvil, rivets and hammer) and provides good protection for head against both sharp and blunt weapons. Its biggest drawback is poor ventilation and air circulation, especially if worn with closed-cell foam padding, making it very hot in warm weather.