Madain Saleh isn't as well-known as Petra, but the Nabateans' second-largest city played a crucial role in their mysterious empire.

As always, our Saudia Airline flight from Riyadh to Medina started with prayer. "Ladies and gentlemen," the flight attendant said over the intercom. "The text that you are about to hear is a supplication that the prophet Mohammed, peace be upon him, used to pray before travelling."The rest was in Arabic. I listened to the record voice, low and ponderous, as I looked out the small window at the unending desert below. I was travelling with friends to Saudi Arabia's hidden desert city of Madain Saleh. While many people have heard of Nabatean capital Petra in Jordan, Madain Saleh, the Nabateans' second-largest city and a Unesco World Heritage Site, remains relatively unknown. Once a thriving city along the ancient spice route, it played a crucial role in building a trade empire. But today its monumental stone-hewn tombs are some of the last, and best preserved, remains of a lost kingdom.

From Medina, we drove four hours to the oasis town of Al Ula, and then continued a little ways further to our hotel in Saudi Arabia's Hejaz province, 1,043km northwest of Riyadh. Our guide Ahmed met us next morning after breakfast. He was tall, lightly bearded and wore a traditional Arabic thobe (robe) and red ghutra (head scarf). Smiling, he told us that he learned his English in New Zealand. As we drove roughly 40km north of the hotel towards Madain Saleh, Ahmed told us about the Nabateans, whose wealth and prosperity came from their ability to source and store water in harsh desert environments. They also held a monopoly on desert trade routes as far southwest as Madain Saleh and north to the Mediterranean port of Gaza. They extracted taxes from camel caravans - laden with frankincense, myrrh and spices - that stopped at their garrisoned outposts for water and rest.

However, in 106 AD, the Nabatean Empire was annexed by the Romans, and Red Sea routes overtook land trading routes. Nabatean cities were no longer centres of trade, and so began their decline and ultimate abandonment. Tucked away in the desert, today Madain Saleh is deserted, silent and stunningly well-preserved. Much of the city still lies under layers of sand. What has been uncovered is a vast necropolis of more than 131 immense tombs. At first, their sheer scale and number was overwhelming. But as we looked closer, the Nabateans' artistry was revealed through carvings of soaring eagles, imposing sphinxes and feathered griffins, not to mention intricate inscriptions. We stopped in front of one tomb, whose inscription translated as being for 'Hany son of Tansy... and descendants', and ended with a date and name: 'April 31AD... carved by Hoor... the sculptor'.

Tomb inscriptions provided insight into the names, relationships, occupations, laws and gods of the people who lived here. The Nabateans left no extensive written history, so these texts, unique to Madain Saleh, are extraordinarily valuable. Ahmed explained that the inscriptions were written in Aramaic, an ancient Semitic language and the lingua franca of the Middle East at that time. Aramaic would have been essential knowledge for business and commerce communication, although the Nabateans also used an early form of Arabic - traces of which Ahmed pointed out in the inscriptions.