It seems that the Baltic is a crossroads that has long supported a high level of culture. My imagination, which seems realistic, sees that culture as much older than the Age of Vikings, older than 'the amber route' traveled by the Romans, older even then the coming of the Celts and Indo-Europeans; though the proto-Italo-Celts may have been fairly early contributors to it.
The Amber Route is the Western equivalent to Silk Road. A difference being that the Amber Route was largely on water.
I suspect that numbers of newcomers with iron technology may have been arriving as late as 800BC, but I also suspect that the culture there was significantly high 2000 years before that and maybe long before that.
There is available evidence suggesting human settlement in Pomerania going back 13000 years.
Some of Europe's, and perhaps the world's earliest man enhanced water-ways(canals) enter the Baltic and connect(or connected)it to the Black Sea and on to Greece, Egypt, Persia, and beyond. It was connected to the heart of the Eur-Asian land mass as well as to the Norse lands, the islands of Britain and Ireland and beyond.
Language students may be the source of much important knowledge of the area.
We know that in recent centuries the languages of the Poles, Swedes, Germans, and Russians too have had their impact on the eastern shore of the Baltic, as well as many others. Much earlier Indo-European languages like the Celtic ones impinged upon them strongly. But traces of much earlier languages remain. There are important traces of language which is not Slavic, German, or even Indo-European.
As late as 300AD, Roman travelers and traders were still noting a people called Veneti. And, indeed, called the Baltic the Venetic Bay (later Vikings would call it the eastern lake). The word Veneti may be related to Venus. It seems to be closely related to Old Irish words for fine, kinship, alliance, tribe, and family. It also seems to relate to Old Norse 'vinr,' meaning, I believe, friend. I think that by this 300AD date the Goths had well begun their conquest of the area.
Peaks the curiosity.