Throughout the Anglo-Saxon period of England and the Viking Age North, the mighty shield wall was a mainstay of battlefield tactics. This defensive use of the shield is how we often imagine shields being used, that is to block and defend against incoming attacks. In the North though, the shield was much more than just a means of defence.
Both Anglo-Saxons and Vikings preferred shield was of course the iconic round shield. With its often colourful and swirling “sun wheel” patterns or in some cases intricate designs, it was a popular choice in the imaginations of young children dreaming themselves as a noble warrior of old.
The round shield itself is obviously much older than the post Roman world, and were used in the North along side the more common elongated type, since at least the bronze age. The Yetholm-type shield being an early example of the Northern round shield (see below). As well as archaeological evidence we also have written records, Tacitus describes the Gothones, Rugii and Lemovii as having such shields,
“the Gothones, who are ruled by kings, a little more strictly than the other German tribes, but not as yet inconsistently with freedom. Immediately adjoining them, further from the coast, are the Rugii and Lemovii, the badge of all these tribes being the round shield, the short sword, and servile submission to their kings.”
We also have examples of Iron age Round shields, like the Wandsworth Shield from England and the Thorsberg shields (Nordic Iron age) from Denmark (see below)
Many Round shields like that of the famous Spartans, have a grip on one side and a place to thread the arm in the centre (see below). This type of design makes the shield much more effective in a defensive situation, giving the user better control over the shield and improved resistance to incoming blows. In the North though, both the round shield and the elongated shield have only one grip positioned in the centre (as seen in the Yetholm-type above and other examples below). This seemingly minor difference drastically changes the way the shields would have been used and their main function.
Back of the Sutton Hoo shield (reconstruction) 
Trelleborg Shield
Despite being less suited to defensive formations like the Phalanx it could still be used in such a way in Anglo-Saxon shield walls. However, having only one central handle makes it a much more versatile tool of war. The single grip forces the wielder to use it far more aggressively, and as the old adage states “Attack is the best form of defence”.
As I am no expert in the martial arts of the ancient north, I will leave you in the hands of someone who is. Roland Warzecha has a YouTube channel that I highly recommend to anyone who interested in the fighting styles of the north. He is a man dedicated to understanding even the minute details of his craft, and has a true passion for his work. The video below is an old video (not on his channel) but where I first saw him about 7 years ago. In it he covers much more than just shield usage and it is a fascinating watch for the ancient warfare enthusiast.
A thought, by having a thin shield, which swivels easily in the hand a Roman pilum would be more likely to be deflected, or if it impacts, it would easily be removed. And it seems this shield was developed at the time of Germanic tribes in conflict with Rome. That would be at a distance, in close contact having a shield where the edge can be used to force an opening it might well be the ideal form to counter a Roman shield wall.
Hullo Jon.The pilums were designed to bend on impact.A deflection might kill 1 of your own men and at such a crucial time ,nobody has the time to free a bent spear from what was basically plywood.Maybe they carried spares to absorb the initial impact and deprive the Romans of some weapons.As an offensive weapon the boss and the rim could be effective up close but like all weapons they’re only as good as the man weilding it.
A deflected spear would not kill as the point would no longer be traveling forward. Its why spears are not thrown sideways. If the shield was thin even if the soft iron of the pilum bent the spear could be easily pulled through the shield making a small insignificant hole.