Complex sentence: King Sverre Sigurdsson and his Birkebeiners confronted Sigurd Magnusson and his islanders, who had besieged Bergen (Bjørgvin) but had not succeeded in taking the castle Sverresborg.
Split into simpler sentences: King Sverre Sigurdsson and his Birkebeiners were against Sigurd Magnusson and his islanders. Who had besieged Bergen (Bjørgvin) but had not succeeded in taking the castle Sverresborg.