Complex sentence: Ptolemy, only thirteen years old at that time, had set up a throne for himself on the harbour, from where he watched as on September 28,48 BC, Pompey was murdered by one of his former officers, now in Ptolemaic service.
Split into simpler sentences: Ptolemy, only thirteen years old at that time, had set up a throne for himself on the harbor. From wtere he watched as on September 28,48 BC, Pompey was murdered by one of his former officers, now in Ptolemaic service.