Right in the middle of the bronze age - they were likely transporting copper and tin ingots. Copper was possibly from Upper Peninsula of Michigan and 99% pure (today’s copper is 2-4%) somebody mined over 231,000 tons out during 3,000BC-1,000BC - not the locals (that would be 4620 rail ore cars at 50 tons each). None of it has been accounted for.
Copper was more valuable than gold as weapons (and more) were made from it. So if those robbers heard about the merchants cargo of copper and tin ingots their was lots of incentive to take it and to make sure no one lived to squeal. For the merchants safety was in numbers, but they just didn’t have the numbers to fend off the likely thousands in the attacking band of robbers.
This was a battle in northern Germany not far from Danzig.
Any copper would have been from European sources. Some Tin was found. Might have been from Wales or The alps.
https://www.sciencemag.org/news/2016/03/slaughter-bridge-uncovering-colossal-bronze-age-battle
https://en.wikipedia.org/wiki/Tollense_valley_battlefield