Shibam, the ancient trading capital of the Yemeni part of the Empty Quarter, has been described as the Manhattan of Arabia. It was declared a World Heritage site by the United Nations. Its tower homes are constructed entirely from the traditional materials of mud and palm wood, with a thin skin of plaster made from locally baked limestone. The city is a living monument to medieval ingenuity. At its center is a mosque. The city was a central point on the frankincense trade route that brought the prized dried tree-sap from the mountains of Oman to the Mediterranean by camel caravan until the time of Christ. The closeness of the buildings keeps the streets and exterior walls cool and shaded for most of the day, and its high exterior wall and labyrinth of streets overlooked by tall buildings provided safety against conquerors.