Complex sentence: Mantua is the ancestral city where the male line of the House of Gonzaga ruled, first as marquesses, then after 1540 as dukes, in vassalage to the Holy Roman Empire.
Split into simpler sentences: Mantua is the ancestral city where the male line of the House of Gonzaga first ruled as marquesses. Since 1540, the Gonzagas ruled the city as dukes in vassalage to the Holy Roman Empire.