Complex sentence: After the ceasefire of Altmark in 1629, the Swedes retained Pillau and set out upgrading its fortifications, constructing a star fort which remains one of the town's landmarks.
Split into simpler sentences: After the ceasefire of Altmark in 1629, the Swedes retained Pillau and set out upgrading its fortifications. They constructed a star fort which remains one of the town's landmarks.