Complex sentence: It protects the Caspian Gates which in turn gave access for the nomads of the northern steppes to the Iranian heartland, and through which Alexander passed on his hasty march to Hyrcania and the east.
Split into simpler sentences: It protects the Caspian Gates, which in turn gave the nomads of the northern steppes access to the Iranian heartland. Alexander the Great passed through the Caspian Gates on his hasty march to Hyrcania and the east.