Great Helm (aka Bucket Helm)
The great helm, also known as the "bucket helm" or "pot helm" is one of the most intimidating helmets in history. Basically, the helm is a strong metal cylinder that encases the head. Visor slits allow somewhat restricted vision, and smaller slits near the center of the helm give the wearer the nice option of actually breathing (as well as providing some minimal amount of cooling).
These helmets were developed in the late 12th century and were especially favored by crusading knights. The helmet provided excellent protection from most weapons and seemed, at the time, to be the perfect complement to full plate armor.
There were only two real weak points to the great helm: The eye slits (opponents would try to knock a knight to the ground then thrust their sword through the visor before the knight could get up) and the flat top, which just screams "Hit me here with that war hammer!" Great helms later in history developed a curved top to better deflect blows from hammers and maces. They also developed visors which could be opened and closed and protruding "beaks" along the nose and mouth to facilitate breathing and comfort. These helmets were typically worn over a chain mail coif and a leather or padded cap.
Great Helms were particularly popular in tournaments. Knights added crests to the top of the helm, like horses or eagles or fists or gerbils, or anything of particular significance to the their lineage or family name (The Kock family of England used a particularly controversial symbol as their crest, one that matched their family motto, "We Stand Erect." King Henry ruled, to the great relief of the other royal families, that the crest had to be changed. I've just been told that there actually was no Kock family in England, and to quote, "stop making up stuff." Okay. So I made the Kock family stuff up. But let me tell you about the royal Ahss family...).
See Strongblade's Great Helm (aka Bucket Helm)