Most of this answer is a copy from when I answered a similar question here.

One item of prestige for great men during the viking era was their personal hird. The hird were the personal guard of a great man. They were full-time professional soldiers who spent their days training, eating and having fun. The larger hird you had, the larger your personal prestige.

Note that before the Scandinavian countries started constructing large churches or castles, a lot of money which would go to the church or castle construction as prestige project would go to the hird.

A hird would be infantry only. While the Scandinavians appreciated horses as a mean of travelling and labour, fighting from horseback was not common at all. It was also hard to bring fodder for the horses on long sea voyages, not even speaking of keeping them calm though rough seas in open longships! Scandinavian horses were almost as small as ponies anyway, and not very useful for charging head-on into an enemy infantry formation.

The hird would be equipped well. Once could expect the men to have chainmail down to the elbow and knee, iron helmets, iron-shod meter-wide round wooden shields, swords or war axes, bows or slings and javelins.

Multiple accounts talk about the vikings throwing stones before combat, although very little evidence survive, I suspect they used staff slings.

The hird would train in formation warfare, like the pig's snout and shield wall formation (the former for offensive, the latter for defensive).

However, the hird would most likely be a minority of the men of a viking host. The vast majority would be made up of odalbönder, free self-owning peasants that left the wife and children (if old enough) or parents (if young enough) to tend the farm while they travelled in viking for plunder and loot. The earliest preserved Swedish medieval laws from the 1200s give bastards (boys born out of wedlock) no right to inheritance - except for weapons and armour!

Hunting was free in those days, there were no King's deer or similar, and most if not all farmers hunted with bow or sling (hunting fowl with sling in the coastal region was common) to increase the amount of food on the table. The equipment standard varied a lot, but a large (meter-wide) iron-shod wooden round shield, a one-handed axe, cloth cuirass and an iron-reinforced leather helmet could probably be expected. These men would plunder chainmail and other armour and weapons on the battlefield to improve their protection and armament. These free men would train with their arms a few times yearly (the thing seem to have been a common place for this).

It seems like the standard was that all men carried both an axe or a sword AND a bow or a sling or javelins, which would make a viking host a flexible force on the battlefield - they can shower the enemy formation with projectiles at a distance and then charge home when their formation is in confusion.

In a home defence situation great men could be expected to arm their tenants, thralls (a type of slave/serf) and others willing to fight for them with spears and shields, but these troops would not be that reliable.

So, let us make a scenario, shall we?

A Scandinavian great man decides he wants to go viking. He starts by making sure his longships are in order. During winter, his thralls burn tar and the women make new sails (a long and laborous process). The great man musters his hird, orders them to increase their training and make sure their weapon and armour is in order. He sends out messages to his allies, friends and extended family. Some are great men themselves, and have their own hirds, some are more or less professional vikings and mercenaries. All reply that they'll show up in spring.

The great man also tells his tenants and allied odalbönder of his project, and they agree to join him, providing a few more communaly owned ships to add to the great man's own.

At the winter thing, the great man will probably publically proclaim his intention, sacrifice to the Norse Gods and have wise men and soothsayers interpret the omens. Omens look good, and many of the odalbönder present at the thing promises to show up for the viking.

Come spring, the ships are fitted, the hirds mustered, stores loaded and messages sent out for those that have promised their presence. More longships show up, perhaps somene far away have heard of the expedition and decided it would be fun to join in, arrives with a set of longships and asks to join the viking.

Finally, the great man's own family, allies and hird and dependent tenants and odalbönder have mustered, say 100 hirdmen and 200 odalbönder and perhaps 50 mercenaries/professional vikings (some of which have experience of sieges and have sailed in the waters they intend to go viking in, whose presence does much to increase morale in the host).

Perhaps 100 hirdmen and 1 000 odalbönder have shown up to take part in the viking, and the fleet, now about 150 longships strong, sail off to France or England to plunder. Along the way they may attract more allies as they sail past (a great host often meant easy plunder as the King of France or any of the various Kings in England would pay ransom instead of fighting), but for the sake of this example, we'll say most others are already sailing in viking or trading.

Once they arrive, they sail up a river, guided by the experienced professional vikings and land every now and then to quickly slaughter small bands of militia and peasant levies that try to gather to counter the invading host. It takes the French (we'll assume they are French now) days to gather a levy large enough to combat the vikings, by that time, they are far off.

However, by the time the viking hos, 1 500 strong, have reached a city with a monastary and an estate of a nobleman, the French have gathered a force big enough to counter them.

Roughly 2 000 French, including the local nobleman and his retainers on horse and fully armoured, the local burghers, decently armed, but not very well armoured and a strong peasant levy (equipped mostly with re-made farmtools, not bad weapons, but still makeshift).

The great man decides he can take this, and forms his host into five great pig's snout triangles and starts to shower the French with stones and arrows. It is not a rain of English longbowmens' arrows like the French will experience 400 years later, but the unarmoured peasant and burger levy suffers, and the French nobleman decides to advance and do something about these annoying archers and slingers.

The vikings then throw javelins, which is devastating. The French lines come into disorder and then the vikings charge, breaking through the French line in two places. The French peasant levy starts breaking and running for their lives. The French nobleman and his retainers charge one of the pig's snouts and break upon the diagonal line of well-equipped hirdmen at the front, who protect the less well-armed odalbönder in the centre. The French nobleman is dehorsed and killed, and the French host starts to rout, while the viking host pursues, killing as many as they can while scattering the rest.

The whole region is now at the mercy of the viking host. Some villages and cities will throw its gates open and let the vikings plunder and rape as they see fit - in many cases they will avoid burning it down if it surrenders. Others, that still have a garrison, might opt to pay a large tribute to be spared, while others will try to resist and be destroyed. Much of the peasantry will flee to monastaries thinking they will be safe there, but the vikings kill monks and peasants with equal indifference and plunder all they can.

As summer starts to end, and other noblemen aproach with their better prepared levies, the vikings load their loot and sail home.

The loot is divided and most come home with a hefty amount.

So, to summarize, yes, the shield wall was a common defensive formation, and the pig's snout was a common offensive formation.

In both formations, the best armoured men fought in the frontline, usually with swords or one-handed axes while those with less armour fought in the second rank, wielding spears over the shoulders of the rank in front of them, while ranks further back pressed their comrades forwards, threw stones (probably with staff slings) or javelins or used bows and waited to take their place in the frontline should anyone in front of them fall or be too wounded to continue to fight.

It has been pointed out that I did not answer the second part of the question - quite right. While it is possible that the vikings learned from the Gauls that fought the Greeks in 280 BCE (but these mostly went east to Anatolia to become the Galatians after being driven out), it is more likely that the Greek phalanx and the Germanic shield wall (many of the Germanic tribes that over-ran the Roman Empire in the 400s used shield wall) are separate inventions - the shield wall makes use of short weapons rather than spears in the front rank, which the phalanx does not. There are however no sources on where the Germanic tribes (of which the ancestor to the vikins were one) learned their shield wall, but I am inclined to believe that they developed separately.

Edit: Thanks for the gold.

Edit2: Corrected a passage about leather armour, since there is no evidence for it (but is for cloth armour). The vikings frequently wore clothes both made from and decorated with fur and soft leather, and I confused it for armour.

I've not seen the TV show in question, but the primary fighting formation of the raiding Norse, known as the Vikings, was the shield wall.

Each man would have a circular shield which would cover himself, and a large portion of the man to his left. With all men linking their shields, the wall would be incredible difficult to break through, and would protect each man from his chin to his upper thigh or knees. If a Viking was wearing a maile haubergeon, which protected him to his shins, he would be very well protected.

It should be noted that the round shield of Viking and Saxon origin was much lighter than the shield used by the Greek and Macadonian Hoplite, and this would not doubt make the formation less rigid and better able to reform in response to a changing battlefield.

I don't know of any evidence this is in any way inspired by how the Greeks fought, or at least not directly. Forming a shield wall is an incredible effective method of fighting as a dense body of infantry, and was used by the Greeks, Romans and even modern riot police. It only truly fell out of use due to the advent of impact warfare, whereby a solid charge of heavy horse could punch through the wall and scatter the infantry within and behind.

It may be that seperate civilisations simple came to the same conclusions over time.

I suppose this is a bit of a follow up question to anyone more knowledgeable.

OPs question about Greek fighting styles got me thinking about the Varangian Guard, the Norsemen from Rus who worked for the Byzantines as bodyguards and mercenaries on the battlefield. Is it possible that whilst being exposed so heavily to Byzantine culture and warfare, these Vikings could have returned to Rus or even gone back to Scandanavia and passed on what they knew? Or are the timings for this too late, I know that the Varangian Guard was formed quite late into the Viking Age, around 980AD if I recall correctly, but it did last as an entity for hundreds of years.

Does anyone have any information on Birkebeinar's in particular fighting in small groups and possibly on ski's during, I think, a battle right outside Trondheim. I seem to remember reading about this, but I can't remember where I have found it, and I unfortunately don't have any of my copies of Sverres Kongesagaer with me.

Can I ask about the cultural and religious significance of Uppsala? Is the depiction made within the TV series realistic? (taking shrooms, multiple orgies, human sacrifice?)