Is this just Television writers having trouble staging the pushing and grinding forces of shield wall combat, or were there actually shield wall tactics in this period where soldiers knelt to protect the legs of their compatriots, forming an almost literal wall?
according to bayeaux, pretty tight:
The shield wall tactics are not entitled only to roman origins, almost any culture that develops heavy shields will develop a close formation to take advantage of them.
Taking in count the influences of roman military in Brittania, and that during the late empire ( ~400) those tactics were still used and adapted to more "barbaric" combat forms like spear + shield
A shield wall formation could be as tight and as complex as the officers in charge would have been able to imagine and to teach, adapted always to their combat style, ofcourse.
Best explanation of a shield wall I've ever seen can be found on YouTube.
It was rows of men standing...no one in their right mind would try one of those triple-stacked "shield walls" shown on "Vikings" or "Last Kingdom," since you'd be immobilized and blinded. The Roman "testudo" was a formation specifically for siege warfare, and it was mobile because the first row of shields was NOT on the ground.
Remember that neither the Danes, nor the Saxons, nor any Germanic force (aside from the Frankish army) were full-time professional soldiers. Any formations used had to be simple enough not to require constant large-unit training, ans flexible enough to react to attacks from different directions. Armies were small (5,000 was a HUGE force), so one would almost always have open flanks. An immobile, complex, blind formation like these triple-stack shield walls would simply not work.
The typical Viking shield would be an awkward tool to put in a wall. They were light weight and held out straight (like one might mimic with a trash can top) ideal for jabbing an opponent... and defensively could easier deflect the force of a blow... as compared to a heavier hardwood or steal shield, which were held by a handle and forearm strap, which had more limited mobility but could absorb more force without redirecting it. Much better choice for a sturdy formation like a true shield wall. That doesn't mean formations of men might not each purposefully cover the guy on their left, but an actual wall only makes sense in fiction.
The circular shields, in my opinion, also were wrong in the depiction of a testudo formation against arrows... which was a Roman formation..
My point is that if you kick into one shield, it won't matter because you kick into 3, since there are nearly 50% of two other shields pressing forward too. That makes the operator of a shield able to work the spear or axe at better and more relaxed conditions.
Since they had axes (as well as spears), the shield wall would benefit them since destroying a shield is best done with an axe.
Their own shields were made of relatively soft wood, light and fairly easily destroyed, but also things get stuck in it and would take energy to both pull out and strike back in. Their axes do huge damage and can also be demoralizing because the pain and energy from each axe blow is felt very well.
I may also add that it's most likely the two shield-men who are behind each side of the front shield that do the attack, as they are only support and can lower their shield to strike hard with their weapon.