For many, Babylon represents excess, greed and sexual licence, but its rich culture gave us the first numbers, law-making and astronomy, writes Mary Russell.

OUTSIDE THE TALL, ornate gates of the British Museum, tourists check their maps, taxis hoot, traffic lights change from red to green. Normality is here. But walk inside to the current exhibition, Babylon: Myth and Reality, and feel your blood chill as you step into the room dedicated to Belshazzar's Feast.

Belshazzar, the last king of Babylon, has invited 1,000 guests to a banquet which is shared with his wives and concubines. In the huge oil painting by Rembrandt we see them drink from gold goblets looted from the temple in Jerusalem by Belshazzar's father, Nebuchadnezzar. Suddenly, a human hand appears and we see it: the writing on the wall, there in front of us, the characters Hebrew, the language Aramaic.

"God has numbered the days of your reign," they say, "and brought it to an end." And with that, Babylon falls.

The Babylon myth in the Book of Genesis says that about 3,000 years ago a distant people started to build a tower that they planned would reach the heavens. Babel, it was called - the gate ("bab") of the god El or Bel, a deity related to our own Bealtaine, which itself means the fires of Bel. Bel also went under the name of Marduk, the god, perhaps not surprisingly, of accounting.

Babylon: did it exist, or was it a mythical place like Atlantis? In the exhibition we see the Mappa Mundi, and there it is - Babylon, straddling the Euphrates. The map, excavated in the 19th century, is made of clay and dates back to the sixth century BC.

One month after the destruction of the Twin Towers, I visited Babylon, driving 80km south of Baghdad to see the ruins of the once-great city. Some of the walls are still intact, the raised bricks depicting lions, horses and dragons.

Children played on a black basalt lion - the Lion of Babylon. This was living history. I had travelled across the Iraqi desert from Damascus and the Arabic word for lion is "assad", the name of the ruling family in Syria, once Iraq's close ally.

Nearby was a large, sprawling building situated high on a hill. It was one of Saddam Hussein's many palaces, designed to be taller than the ruined Babylon - an aspiration mirroring that of the original builders and a terrible mistake as it turned out.

According to Genesis, the Tower of Babel was built by Nimrod, great-grandson of Noah. It was rebuilt by Nebuchadnezzar and it is his reign (605 BC-562 BC) and its aftermath that this exhibition covers.

FOR MANY, BABYLON represents excess, greed and sexual licence - such images graphically presented both in the Bible and by artists, musicians and writers down the ages.

One of the most poignant of the Psalms is 137:1-14, "By the waters of Babylon, we lay down and wept", which gives voice to the exiled Jews, their temple in Jerusalem destroyed and they themselves forced to trek across the Judean desert to Babylon, where they were held in captivity.

For this crime, Nebuchadnezzar is portrayed by William Blake as a crazed man crawling on all fours and covered in hair; a creature lower than the beasts of the earth, doomed forever to be the fall guy for the righteous, his city's reputation worse than that of Sodom and Gomorrah.

But there are other ways of looking at Babylon. Its culture has given us many firsts, among them numbers and law-making. It was the Babylonians who laid down a system of astronomy which travelled eastwards to India and westwards to us via the Greeks; a system which allowed them to predict the movement of the planets.

They also developed the sexagesimal system whereby they counted in 60s, which we still do when measuring time. The number 60 was chosen because it is divisible by 12 other numbers.

The clay tablets explaining these number systems, and many more, are all on display at the museum, including a model of a sheep's liver and gall bladder used in divining whether an animal sacrifice was acceptable to the gods or not.

Brick-making was a breakthrough for the Babylonians. Previously, builders had used stone, but the new process involved clay, with tar used as mortar. That discovery led to many marvellous works involving striking blue-glazed bricks carrying depictions of lions and dragons. The most famous example of this is the Ishtar Gate, the huge, lavishly decorated ceremonial gate which led into the Processional Way, along which the gods, king and court passed during the major new year festivities.

The original is in Berlin, but parts of it have been loaned to the exhibition: the proud, strutting bull and the mythical dragon, or mushussu, its tail alone made up of eight glazed bricks. The dragon was the favoured creature of the god Marduk.

The king Hammurabi, who pre-dated Nebuchadnezzar by some 1,000 years, made Babylon the capital of a number of Babylonian kingdoms. He laid down a set of laws - the Code of Hammurabi - and embraced the idea of transparency by having them written in stone and put on view so that all might read them (though literacy was far from universal). The stone was excavated just over 100 years ago and is at the British Museum on loan from the Louvre.

We learn much about Babylon from the Greeks and from the Old Testament, though much in these sources overlaps, with real and mythological figures blending. For instance, the legendary demi-god warrior, Queen Semiramis, is sometimes credited with the building of Babylon, and it seems her character was based on a ninth-century BC Assyrian queen, Sammuramatt. On display is Queen Sammuramatt's good luck stone, inscribed to Ishtar.

AND WHAT OF that most elusive of the Seven Wonders of the World - the Hanging Gardens of Babylon - the only one never to be found? Thought to be so-called because of the challenge faced in growing plants on high terraces, we do at least have a list here of what was grown there. It includes coriander, beetroot and alfalfa.

In 539 BC, Babylon fell to the Persian king, Cyrus the Great, and one of his acts was to release the Jews from their captivity. Though many returned to Jerusalem, many also elected to stay, so that the subsequent cultural history of Mesopotamia has a significant Jewish input.

Babylon persists as an example of depravity, its name used to describe any and every type of excess and oppression. For some, Rome and/or the Catholic Church, with its incense, candles and its princes dressed in crimson silk, is seen as the Whore of Babylon. Hollywood made numerous films about it, some of which are on video at the museum.

In a different kind of video, Jamaican professor of sociology Barry Chevannes tells us that, in Rastafarian culture, Babylon is a symbol of white power and the excesses of an oppressive state. Rastas often use the word "Babylon" to describe western society.

This exhibition does not refrain from reminding visitors of the senseless damage done to this great ruined city by an army of occupation whose ill-educated military dug anti-tank trenches and human waste channels through archaeological sites and damaged the Processional Way by driving heavy lorries along it.

The British Museum's is truly a must-see exhibition, for we will not see its like again in this part of the world.

• Babylon: Myth and Reality, supported by the Blavatnik Family Foundation, is at the British Museum, London, until Mar 15. Admission is free but booking is advisable at www.britishmuseum.org