Complex sentence: The autonomy and Greek institutions of Seleucia were respected since the Parthians were thorough philhellenes, immediately facing the city on the left bank of the Tigris the Parthians founded Ctesiphon, the new capital of the new empire.
Split into simpler sentences: The autonomy and Greek institutions of Seleucia were respected, since the Parthians were thorough Philhellenes. Immediately facing the city, on the left bank of the Tigris, the Parthians founded Ctesiphon, the new capital of the new empire.