The earliest known weapons “buyback”?
They may have been getting ready for some **** to go down, but more likely they were a trading entrepot. During the Roman Empire, there was a big uptick in iron ore processing and iron metallurgy around the Baltic.
Also, it's little known, but during the Crisis of the 3rd Century, the Romans were still gettin' it done, and went scorched-Earth on Jutland, and apparently put a garrison at the strait, to control traffic and trade.