When Julius Caesar arrived in Alexandria in 48 BC and sided with Cleopatra's faction, Arsinoe escaped from the capital with her mentor Ganymedes and joined the Egyptian army under Achillas, assuming the title of pharaoh. When Achillas and Ganymedes clashed, Arsinoe had Achillas executed and placed Ganymedes in command of the army. Ganymedes initially enjoyed some success against the Romans, negotiating an exchange of Arsinoe for Ptolemy, but the Romans soon received reinforcements and inflicted a decisive defeat on the Egyptians.
Arsinoe was transported to Rome, where she was forced to appear in Caesar's triumph. Despite usual traditions of prisoners in triumphs being strangled when the festivities were at an end, Caesar spared Arsinoe and granted her sanctuary at Ephesus. Arsinoe lived in the temple for many years, always keeping a watchful eye for her sister Cleopatra, who saw her as a threat to her power. Her fears proved well-founded; in 41 BC, at Cleopatra's instigation, Mark Antony ordered her executed on the steps of the temple. She was given an honorable funeral and a modest tomb.
Octagon was a vaulted burial chamber placed on a rectangular base with the skeleton of a 15 or 16 year old woman in a marble sarcophagus. According to an interpretation Octagon was a monument to Ptolemy Arsinoe IV, the youngest sister of the famous Cleopatra VII,was murdered in Ephesus in 41 BC.
Arsinoe IV (ca. 68/67 – 41 BC) was the fourth daughter of Ptolemy XII Auletes, sister of Ptolemy XIII and Cleopatra VII, and one of the last rulers of the Ptolemaic dynasty of ancient Egypt. When their father died, he left Ptolemy and Cleopatra as joint rulers of Egypt, but Ptolemy soon dethroned Cleopatra and forced her to flee Alexandria.