Complex sentence: He settled the citizens of Pest on a hill on the opposite side of the Danube; their new fortified settlement, Buda developed into the most important center of commerce by the end of Béla's reign.
Split into simpler sentences: He settled the citizens of Pest on a hill on the opposite side of the Danube. Their new fortified town, Buda developed into the most important center of commerce by the end of Béla's reign.