Vastly overlooked and simplified, the use of the shield in medieval European martial arts is nearly as vital a skill as the use of one's weapon. Before the entrance of plate armor, shields were crucial to any hope of survival in battle as even the best coat of maille could be pierced or hewn with enough force. Thus nearly all soldiers wore a shield strapped to their left arm, sometimes their sole protection.
The incredible structural integrity of the shield is also little-known. Movies have shown these cloven in half at the first blow and discarded, while the real thing was reinforced with two or three layers of boards running in perpendicular directions and cased in an iron rim. Some shields were passed down for generations alongside the weapons they accompanied.
Like all implements of war, the shield evolved to meet different needs during the Middle Ages and changed shape and constrution. These can be broken into four types:
The Oval- ca.500-1000 C.E.- These were used as far back as Classical Greece and could be seen in the Low Middle Ages as much of Europe had fallen back into post-Roman disarray, to greatly simplify the situtation, and used the styles of their occupiers.
The Round/Teardrop- ca.500-1200 C.E.- This variety started with the Round shield, which was widely used by Vikings and Anglo-Saxons. It eventually grew a tapering point that could efficiently guard the lower half of the body without necessitating a costly drop of the arm.
The Kite- ca.1000-1300 C.E.- As the Round shield tapered into the teardrop, the Kite continued this trend. It could easily block a strike to the legs with a twist of the arm and stood to the collarbone of the wielder. These were depicted in large numbers in the Bayeaux tapestry, which depicts the Norman invasion and the battle of Hastings in 1066.
The Heater- ca.1250-1500 C.E.- Plate armor was widely worn when the Heater style made its introduction and the large, body-sized Kites were quickly becoming needless. Heaters are the type commonly seen as the base of a coat-of-arms with the flat top and quick point.
But why did they make this new type instead of just using the Round shield which suited the same needs? The answer deals with a technique soldiers were running into when combatting edged weapons. When the opponent would bring down a vertical strike, the blade, be it sword or axe, would stick into the wood of the shield if it had no iron rim. This could disarm or otherwise manipulate the enemy into a vulnerable position. It it because of this that the tops of the shields had a broad, flat top that could easily intercept such a blow, where the rounded edge would allow it to glance off.
And then there's the problem of holding onto the shield. The earliest method was to have a handle directly in the center and have a raised metal dome on the face, called an Umbo. This was a relic of Roman and tribal shields and were commonly seen on Oval and Round shields. However, this restricted the grip of the shield to just the hand and allowed it to spin around the handle's axis when struck. This was solved by bolting cloth or leather straps to the back into which the entire forearm was fastened. Additionally, all shields had a strap running the length of the device, called a Guige strap, which a soldier could loop around his shoulder or torso for marching.
But alas, as must be covered with most historic topics, the downfall. Once again, plate armor heeds the call! Shields simply weren't needed when you could used your steeled arm to ward off a blow. This let larger swords like the Bastard and Two-handed swords be used on their own without a bulky plank of wood to get in the way...
Resources:
Medieval Swordsmanship, by John Clements.