Complex sentence: Settlements in the area existed from the 6th century onward, when during migration period, Slavic tribe Ślężans settled on the Oder and erected on Ostrów Tumski a gord.
Split into simpler sentences: Settlements in the area existed from the 6th century onward, during the migration period. A Slavic tribe Ślężans settled on the Oder and erected on Ostrów Tumski a gord.