Review Detail of Armand_Targaryen in GOT : Reborn as a Frey

Something that I have always wondered is if the Valyrians conquered the continent of Essos because all the noble families of Valyria died? It just doesn't make sense, who ruled the big cities in Essos? the slaves? Weren't there dragon lords ruling those cities? Didn't the families of Valyria have their own territories?