Complex sentence: Romans still called their state "Res Publica" in the era of the early emperors because, on the surface, the organization of the state had been preserved by the first emperors without significant alteration.
Split into simpler sentences: Romans still called their state "Res Publica" in the era of the early emperors. Because, on the surface, the organization of the state had been preserved by the first emperors without significant alteration.