Complex sentence: Raynald used the castle to attack the rich caravans that had previously been allowed to pass unharmed; he also built ships there, then transported them overland to the Red Sea, planning to attack Mecca itself.
Split into simpler sentences: Raynald used the castle to attack the rich caravans that had previously been allowed to pass unharmed. He also built ships there, then transported them overland to the Red Sea, planning to attack Mecca itself.