RIYADH - Even in a Google-mapped world, Saudi Arabia is such an unseen kingdom that very few outsiders have an inkling of its wintry peaks, coral reefs and stylish cafes.

Its best-kept secret is Madain Saleh, the ruins of a lost city akin to Jordan's Petra, but still blissfully crowd-free. These rose-red sister cities deep in the desert were the jewels of powerful Nabatean merchants - of Arabic origin - who traversed the ancient Incense Route profiting richly from the frankincense trade.