If you are calling longships, birlinns, etc. galleys, that’s fine.
But the previous poster was talking about Roman galleys
A practical explanation is fairly simple: most of Northern European kingdoms were simply not naval powers, and their ships were not designed principally for raiding or even warfare, but rather for cargo transport. A Viking Longship, though it may seem simple, is extraordinarily practical given its relatively cheap cost and extreme suitability to the kind of usage it was put to. Moreover, Scandinavian mariners built their knowledge of how to sail over generations. Retaliation would have been impractical considering the difficulties of even figuring out where to attack, much less moving a significant force to do so.
However, Northern Europe developed its own kind of fighting ships that completely outclassed the longship, and that was a factor among many which led to the end of the “Viking” age.
One question I have long been curious about was why there was no invasion of Denmark from the south. Frankia bordered southern Denmark but it doesn’t seem like they made a serious push to attack there, evne though this might have been a practical response. Eventually, of course, Denmark converted to Christianity and ceased to be a threat so the point may be moot.
See this article:
The Danevirke consists of several walls, trenches and the Schlei Barrier. The walls stretch for 30 km, from the former Viking trade centre of Hedeby near Schleswig on the Baltic Sea coast in the east to the extensive marshlands in the west of the peninsula. One of the walls (named Østervolden), between the Schlei and Eckernförde inlets, defended the Schwansen peninsula.
According to written sources, work on the Danevirke was started by the Danish King Gudfred in 808. Fearing an invasion by the Franks, who had conquered heathen Frisia over the previous 100 years and Old Saxony in 772 to 804, Godfred began work on an enormous structure to defend his realm, separating the Jutland peninsula from the northern extent of the Frankish empire.
I’ve heard of it, and I don’t think that answers the question. Fortifications, especially on such a limited scale by people not schooled in siege tactics*, are not just free passes that make attack impossible. Much of that line was not really defensible Furthermore, the Franks had sizable of military resources if they cared to exercise them, which is one reason the the split Kingdom gave rise to the two great powers of the Medieval era. The Wikipedia article also suggests that military reasons for the barrier were also secondary, which is actually new information to me.
*Sieges remained a weakness of the Scandinavian method throughout the “Viking” age. It didn’t mean they always failed but it wasn’t a specialty and wasn’t necessarily suited to their armies or technology. Fortification was a common response to Viking raids and over time became another factor that really hindered their operations. It’s also probably no accident that adapting Frankish methods of war into their arsenal allowed the Normans steamroll the Anglo-Saxons.
Limited scale? 30km of defensive walls - up to 6m high, with palisades, heavy stone walls, deep, wide ditches and moats - is not a ‘limited scale’ by any standard.
These walls were continually maintained and upgraded over a period of centuries, before, during and after the Viking age. If they incorporated canals for shipping that would have been a big incentive to keep them in good order.
Defending fortifications does not require ‘siege tactics’. Defending fortifications is easy, attacking fortifications is difficult - that’s the whole point.
Lengthy fortifications such as those are inevitably weak in some areas, and the fact that they had to be rebuilt periodically suggests there were long periods when they were not in good repair at all. Second, the fortifications weren’t necessarily that imposing even in their own day. Finally, while you don’t absolutely have to have a good understanding of siege warfare to defend walls, it does help because there are many tactics which can break those defences and which require counters, which were well-documented in Roman sources and which we know were understood in the Carolingian Empire, for instance.