I think I will just retreat towards Ektabana and Susa, build walls, raise Thorakitai and advanced armies later on and defend the crossings and my larger cities for now. The cities on the mediterranean are more profitable and easier to conquer, anyways. Means giving up 6 or 7 provinces, though. But if I sell all buildings, I can raise so much money to invest in the west and in stone wall defenses elsewhere.