They were some of the most feared and lethal warriors of their time, Scandinavian raiders who were experts in navigation and mobility, armed with iron weapons and advanced tactics, who would bear down on other European settlements for loot and pillage. Vikings were terrifying for all those not protected by high walls or standing armies. For victims of these raids, death could come quickly and with little warning. The Vikings would raid deep inland by taking their longboats upriver, meaning that death could always be lurking just around the next bend. Towns on the coast were more likely to be raided, but they could at least see ships approaching on the horizon.
Since Vikings could barrel down at around 10-11 knots, though, that only gave them an hour of warning, Not long enough to marshal a defending force, but long enough to crap yourself once or twice and maybe say a few confessions.
Smart victims would then cower and hide, allowing the village to be plundered without resistance or they might even drag valuables out and buy off the Vikings. This might sound like cowardice, but the Vikings were professional raiders who worked hard to ensure that they had the upper hand, partially through reconnaissance.
Yeah, by the time you saw the Vikings, they probably already had a whole dossier on you, complete with whatever it is you did with those kind ladies in the expensive inn.
The Vikings actually took plenty of time to conduct quiet observation when they could before a raid, making sure there weren't a bunch of enemy warriors that happened to be in town. Once they were sure it was just you and a few farmers and craftsmen around, they would launch their attack, keeping their men in tight formation and eradicating serious resistance before it could prepare.
This was made all the easier for the Vikings by how they organized their forces, employing ranged and melee attacks. Yeah, the Vikings basically had a combined arms team. They rarely had cavalry, though.
Viking raiders carried personal weapons and weapons provided by their magnate, a sort of chieftain. Younger and poorer raiders would usually carry an ax from home or a hunting spear, weapons made with mostly wood and a little iron. Shields, made of wood, were easy to get as well. Bows were relatively rare, but available.
Richer or more established raiders were likely to carry a sword and might even have chain mail or other iron armor, making them extremely challenging to kill for startled farmers in England or France.
Archers and spear men would engage any brave defenders as soon as they got into range, and swordsmen and raiders equipped with axes would charge forward with shields for protection.
So, yeah, unless the Vikings stumbled into a fight with the king's army because of some bad intel gathering, they were going to win. Every once in a while, they'd do something bold like besiege Paris, and even then they'd usually win, because, again, great intelligence and professional are raiders are typically victorious.