Pompey's Pillar

Pompey's Pillar stands on a low hill in the Karmouz district of Alexandria, visible from a long way off across the rooftops. It is a single shaft of red Aswan granite, roughly twenty-seven metres tall with its base and capital.

It is the tallest ancient monolithic column of its kind still standing where it was raised, and it is almost the only thing left of the building it belonged to.

For the scale of what is missing. This hill was the Serapeum, the great temple of Serapis and the religious centre of Greek and Roman Alexandria. It held a daughter collection of the Library of Alexandria. It was destroyed at the end of the fourth century, and what you are standing in is the footprint. One column, two sphinxes and a scatter of granite is what four centuries of the most cosmopolitan city in the ancient world came down to.

The underground parts are the surprise, and most visitors miss them. There are rock-cut galleries and chambers below the platform, including a Nilometer and passages associated with the sacred Apis bull. You can go down into them.

And the name is a good story in itself. The column has nothing to do with Pompey. Its Greek inscription, on the base, dedicates it to the emperor Diocletian around AD 300. The Pompey name came from medieval visitors who assumed the great general's ashes were in a pot on top of it, and it stuck for eight hundred years.

The Serapeum was founded under the Ptolemies for the cult of Serapis, a deliberately synthetic god combining Egyptian Osiris-Apis with Greek attributes, created to give the city's Greek and Egyptian populations a shared object of worship.

The founder was Ptolemy III Euergetes, who reigned from 246 to 222 BC. Bilingual foundation plaques in Greek and hieroglyphs, recovered when Alan Rowe excavated the hill in the 1940s, record the dedication in his name. The complex was rebuilt on a larger scale in the Roman period.

The column was raised around AD 300, according to the dedication cut into its base, in honour of Diocletian. It has stood on that spot ever since.

In AD 391 the temple was destroyed during the campaign against pagan cults in the city under the patriarch Theophilus. A church was raised on the ruins, and the ground was quarried and built over for centuries afterwards. The hill itself is not a heap of rubble, though: it is the natural rock ridge of old Rhakotis, buried under later occupation debris — which is why the galleries and cisterns beneath the platform could be cut into solid stone. The monument's own Arabic name is Amud el-Sawari, the pillar of columns.

The galleries under the platform were opened up in the 1890s by Giuseppe Botti, the first director of Alexandria's Greco-Roman Museum. What came out of them included a black stone statue of the Apis bull, of Roman date, now in the city's museum collection.

• The column itself, still standing in its original position
• The Corinthian capital at the top, which is thought to have carried a statue of the emperor
• Two granite sphinxes on the platform beside it
• The underground galleries and chambers below the site
• The nilometer cut into the rock
• The view across Alexandria's rooftops from the hilltop