Nabataean tomb in Mada'in Saleh (aka Hegra), Saudi Arabia

© Tuul & Bruno Morandi/Getty Image

The lonely castle. Mada'in Saleh, Saudi Arabia

About 2,000 years ago, Mada'in Saleh, or Hegra as the Romans called it, was a thriving city of the Nabataean Kingdom, and a center for the trade of spices, incense, and myrrh, a valuable tree resin used to make perfume and medicine. The Nabataeans were Arab people whose precise origins are unknown. They lived in northern Arabia and the Southern Levant. Mada'in Saleh was their second-largest and southernmost city after Petra, their capital city to the north. About halfway between Petra and Mecca, Mada'in Saleh served primarily as a trading crossroads and was instrumental in establishing the Nabataeans as prosperous middlemen to the rest of the ancient world.