Complex sentence: During this period a castle called Östhammarshus was built, but already in 1434 the castle was burnt down by its Danish vogt, who fled to Denmark over the sea when faced with the Engelbrekt rebellion.
Split into simpler sentences: During this period a castle called Östhammarshus was built. In 1434 the castle was burnt down by its Danish vogt, who fled to Denmark over the sea when faced with the Engelbrekt rebellion.