Complex sentence: Arriving in the town of Sölvesborg, they find it surrounded by barbarians who crossed into the Empire when the Great Northern Ocean was frozen over during the previous winter, and who are preventing the town from resupplying the nearby Fort Nogersund, which the barbarians are besieging.
Split into simpler sentences: Arriving in the town of Sölvesborg, they find it surrounded by barbarians who crossed into the Empire when the Great Northern Ocean froze over during the previous winter. The barbarians are preventing the town from resupplying the nearby Fort Nogersund, which they are besieging.