I've seen this same conversation come up repeatedly, in RPG gaming communities, LARPers, SCA, movie fans... hopefully /r/AskHistorians can give me an answer with some credibility.
There are two general schools of thought, that shields were strapped on to the arm with two straps, or held in the fist at the center. I know both of these likely happened (I'm sure there is an example of just about everything in history) but in general, what was the popular way to hold a shield? Did it change over different periods in history? Did it vary with different types of shields (I'm certain no one was strapping a buckler to their arm, but what about kite shields, tower shields, etc)?
What's the right answer?
Comment I made from his post over at /r/wma
"Although there is not alot of extant historical material from the "dark ages" about the use of the sword and shield, we can extrapolate on their use from Renaissance sources. The early historical martial arts manual is MSI.33 from the late 14th century which depicts the use of the sword and buckler. From their illustrations and descriptions they held the buckler in the fist. Other manuscripts, surviving artifacts, and modern reconstruction, greatly support this notion. They were very rarely strapped to the forearm. The later masters from Bologna, Italy in the late 16th century also wrote extensively on the subject of sword and buckler. In which they describe holding the buckler in the fist and having it well extended at or around shoulder height. Having the buckler in this extended postion, creates a virtual cone of defense in which one only needs to move the buckler slightly in order to close off a line of attack. It is an active means of defense, for one moves the buckler in relation to the sword (and that of the opponent) in order to compensate for vulnerabilities created from the actions of your sword.
Many times you see people who are unfamiliar with historical technique hold their buckler too close to their chests. This essentially eliminates the cone of defense, making the buckler a poor means of defense. Roland Warcheza is an expert in the MS I.33 style and applied the principles from that style to the use of the center gripped viking shield, shown here. This is, in my opinion, one of closest reconstructions of Viking sword and shield fighting you are going to find. In this example you see that the shield is held with the arm well extended, which allows one to cover nearly the whole body with relatively small movements.
When it comes to forearm strapped shields, we can again turn to the Bolognese masters of the 16th century. While they didn't have heater shields they did have a rotella, a round shield about 20-30inch in diameter, that is strapped to the forearm. This was often accompanied by the sword or a spear. If you look at the picture in the article linked above, you see that the arm will again well extended. In such a guard you are again creating a cone of defense. With the rotella held in such a way, you completely close off the left side, and while providing some cover to your right. Again one would have to simply move the arm up or down to protect the head and legs. I imagine the same would be true with a heater shield, however, as I understand it a heater shield was mainly for mounted combat, and it's shape would not provide much protection to the legs.
Many time you see people holding their forearm strapped shield against their chests, which eliminates the cone of defense and only provides protection to the area immediately behind the shield. Such a position also requires much larger actions (and as a result, slower) to cover your head and legs.
All this being said, there was a article recently, in which forensic analysis was performed on skeletons from known battle fields. It was found that strikes to the head and lower legs were exceedingly common. Perhaps that is why you often see ancient warriors depicted with prominent helms and greaves, however I don't know how historically accurate that would be."
Have you tried r/WMA yet?
"Tower shield" is a modern term most likely invented by roleplaying games, the historical counterpart would be the pavise. It was used mostly by missile troops that would use it as mobile cover.
Here's a short overview of shield usage in ancient and medieval period: http://www.myarmoury.com/feature_shield.html It seems to be well sourced.
Finally, my topic! :D
Since renessaince combat, pavisses and bucklers are already covered, I'll do the viking roundshield.
A normal germanic/viking/norman/slavic roundshield was a round (or in same cases, oval) shaped piece made of wood. In the center of it (again, in most cases) was a bowl-shaped iron thingy called umbo.
The shield itself had a hole in the center, which allowed one to put his hand through the shield, and into the umbo, and firmly grab the wooden(somethimes thick leather) handle that was just behind it, on the inside.
In a shield-wall, and generally, during a battle, a warrior would hold his shileld in front of his body, about a foot away from his chest, and locked with the shields on his left and right. In some cases, he would lift the shield up to the level of his eyebrows, and tilt it 45* backward, when he was maiking the "roof" of the shield formation.
In a 1x1 combat, the shield was held slightly to the side, at a 45* angle, either inwards (of the warrior was going for a short strike with his sidearm) or outwards (if he wanted to take a full, diagonal swing, or stab with a spear).
Holding your sheild straight with the line of your shoulders, or worse, close to the chest/arm was a very bad idea, since wooden shields were comparatively weak, and the art of defense depended on letting your opponent's attacks slide off the shield, not bite in it at a straight angle.
As a side bonus, this not only protected the shield from being shattered, but also, every time your opponent's attack slid off to the side, his neck, right arm and face would be momentarily exposed to counterattack.