From ancient times until the late Middle Ages, a shield was a soldier’s standard equipment. During the years from the 11th to the 15th century, shields evolved side-by-side with armor and other weapons.
Many people think medieval shields were made of metal, but the reality is that metal was too scarce, expensive, and heavy for this purpose. Shields were actually made of planks of wood bonded together and reinforced across the back by strips of wood or metal. Over the course of time, the planks of wood were replaced by layers of thin planks of wood, with the grain turned 90 degrees from the preceding layer; the layers when glued together took advantage of the natural strength inherent in the wood grain, much in the same way plywood is made today. Metal staples and fasteners were added to attach the straps and the grip, and perhaps a metal edge to deflect cuts. Often, shield-makers would cover the front of the shield in cloth.
From the fall of the Roman Empire until the 11th century, shields in Europe were circular or oblong, with a central grip on the inside and a dome covering on the outside (known as a boss). The longevity of this shape was likely due to its practical application in combat on foot.
Mid-11th century, the shape elongated into an inverse teardrop; modern historians refer to this shape as the “kite shield”. The Norman cavalry used this shield to cover the left-side of the body from shoulder to ankle. Being rather unwieldy due to its size and bulkiness, arm straps were added to the hand grip. Added support also came via a body strap that wrapped over the shoulder and under the arm, thus allowing a knight to relax his arm.
In the 12th century, the kite shield lost its rounded top in favor of a flat, straight edge. The advantage to the knight was the ability to hold the shield high without compromising his field of vision. The late 12th to early 13th century produced a smaller version that modern historians call the “heater shield”, since it looks like the bottom of an iron or “heater”. The other significant characteristic is that the shields’ body changed from flat to curved, to better enclose the knight’s side. These smaller shields, about three feet in length, were easier to use in combat and remained in use until the mid-14th century.
At that point, the progression of plate armor made shields obsolete, although knights still used them in tournaments. There were two other types of shields used in this later time period through the end of the Middle Ages: the buckler and the pavise. More on those later.
Reference: Daily Life in the Middle ages, by Paul B. Newman
Here’s a website showing shield construction: http://www.yeoldegaffers.com/project_shield.asp