AstralLein
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These are just opportunists. They don't care that the people of Fortress regularly raid, burn, and kill them. Let them trade. 
Or maybe they only trade with the loyal jarls. Who knows. Or maybe there are jarls who are traders and have moved away from the concept of raiders and plunderers.
Or maybe they only trade with the loyal jarls. Who knows. Or maybe there are jarls who are traders and have moved away from the concept of raiders and plunderers.