There are, in fact, two types of war-hammer (or warhammer). the first is that of the standard big hammer
. It's between about 1'
haft with a head that'd be 4-8" inches
long, and either a "proper" trapezium- shaped or cubed
They were often made from steel, but cheap ones could have stone or lead heads, and were often confused with maces- but a mace has a foot long shaft and a head that is a large sphere (or similair) of steel; sometimes, maces would be spiked. The mace is, in turn, confused with the morning star (ball on chain)...
While popular in this form, the Warhammer was nothing special. It hit things, it broke things, it hurt things. It was the generic, all-purpose hitting thing; Like a club, but harder, and more- stylish.
The other form of warhammer is a weapon that's basically a hammer, about 4' reinforced long haft... with a pick in the head. That's right. This warhammer proper is not a hammer, it's a pick axe. the point is normally about six inches on the 1' by 4" head of metal; These weapons take immense strength to use properly, but can be used in both hands.
Though unwieldy- it only took one average blow to kill a healthy man. The disadvantage is the momentum, and the big pick means that it can get stuck. Also, the massive damage and weight meant that men could be sent flying- both by being struck by it, and by wielding it.
Walls and doors were a small matter. It's possibly one of the most damaging weapons ever, and caused more damage on a blow than a decent halberd or nageyari.
A single good blow from the pick-headed warhammer will kill. Armour? Punctured, then dented, then shattered. Bone? Crushed.
The warhammer saw quite a bit of use, in both forms; the Vikings created the first, the latter was created later by an unknown source, but was popular amoung mounted warriors and those who wanted to prove their strength. The rich often used them in this way, as well as finely decorating them.
Lords who used it often used it with a leather harness or such to stop it slipping out their hands; Others chose not to, to stop them slipping out their saddle.
Though a common weapon, the first was too ineffective for good use in open combat; Nonetheless, the Germanics still tried, as did much of the French armies during the 13 century. The second was used mainly by those who rode mounted, and was used all over the known world for a long time.