Long before Alexander arrived on the scene,  this hill was the citadel of Rhakotis, dedicated to the worship of Osiris. The Ptolemies in their turn contracted a temple of Serapis on its summit. Here, with a collection of around 200,000 manuscripts given to her by Mark Anthony, Cleopatra endowed the second great Alexandrian library, which was to remain attached to the Serapeum until the temple itself was destroyed by a mob. Unbelievable as it seems now, this was the most learned spot on earth for almost a century.

When European travelers arrived in the 15th century the pillar caught their attention.  they wrongly named the pillar after Pompey, believing fables which said that his head was enclosed in a ball at the top. 

It actually has nothing to do with Pompey, according to an inscription on its base, It was dedicated in c. AD 297 by the prefect of Egypt to the emperor Diocletian (1) for his victory in AD 296 over the Christian Achilles, who had usurped the title of emperor.

On the west face of the granite base, the following four lines in Greek characters can still be deciphered: "To the most just of emperors, the divine protector of Alexandria, Diocletian the invincible: Posthumous, prefect of Egypt." It was set up in honor of Diocletian, who supplied food for the starving population after the siege of the city. and it may once have had an equestrian statue on top which would explain its Arabic name; `Amuwd al-Sawariy.

`Amuwd al -Sawariy was the, most important surviving element of the Serapeum, the ancient temple of Serapis, which certain ancient writers described as the most beautiful in Alexandria and second only to the Capitol in Rome. 

The temple was built on a platform reached by a flight of one hundred steps. In the sanctuary was a richly decorated statue of Serapis, a Graeco-Roman deity combining the attributes of Osiris and Apis. Excavations carried out in 1943-4 in the southeast and southwest corners of the Ptolemaic enclosure yielded remains of the foundations of the temple containing plaques made of gold, silver, bronze, opaque glass and ceramic tiles which bore the inscription in Greek and in Egyptian hieroglyphics: "King Ptolemy [III Euergetes], son of Ptolemy and Arsinoe, divine siblings, [has dedicated] the temple and sacred enclosure to Serapis".

The Serapeum was destroyed toward the end of the century during the reign of Trajan (98-117) and a new temple was probably built in the following decade, during Hadrian's reign. In AD 391 the Christians destroyed it and built a monastery and a church dedicated to Saint John the Baptist, which in turn did not last.  The remains of Roman baths and various statues of the Sphinx and Ramsiys II brought to Alexandria from Heliopolis (date unknown) can still be seen on the site of Pompey's Pillar. The function of the two long, underground galleries set with niches to the west of the pillar is not known for certain.

Some historians believe that "Pompey's Pillar" was part of the  portico of the "sister" library of the Great Alexandria Library housed in the Serapeum. The 88 foot pillar 
(including the base) is made of pink Aswan granite and tapers slightly from the 9-foot diameter of its base to one of 71/2 feet at the top.
 

Roman Emperor born of parents who had been slaves, at Dioclea, near Salona, in Dalmatia, A.D. 245; died at Salona, A.D. 313. 

He entered the army and by his marked abilities attained the offices of Governor of Mœsia,  consul, and commander of the guards of the palace. In the Persian war, under Carus, he  especially distinguished himself. When the son and successor of Carus, Numerian, was  murdered at Chalcedon, the choice of the army fell upon Diocletian, who immediately slew   with his own hand the murderer Aper (284).